No subject
Dimitrios Papadopoulos
chaosdynamics at googlemail.com
Tue Dec 2 15:14:20 GMT 2008
Δεν το πιστεύω. Εκεί που λέγαμε ότι το θέμα έχει κλείσει το
συνεχίζουμε. Κι εγώ μαζί με σας. Αφήστε να πάει στο καλό. Τι ξύνετε
πληγές; Δηλαδή δε μπορούμε στην Ελλάδα να μη φαγωνόμαστε;
Για κάθε καβγά που θα ξενάει στη λίστα θα λαμβάνετε ως δώρο από μένα ένα σπαμ:
Καλή διασκέδαση
Thanks for making this the number #1 page in Google on 'sed'!
Sed - An Introduction and Tutorial
Bruce Barnett
Last update: Sun Jul 20 21:59:55 EDT 2008
Thanks to Keelan Evans, Fredrik Nilsson, and Kurt McKee for spotting some typos.
Thanks to Wim Stolker and Jose' Sebrosa as well.
Thanks to Olivier Mengue.
Thanks to Andrew M. Goth.
Thanks to David P. Brown.
Thanks to Axel Schulze for some corrections
A big thanks for Fazl Rahman for spotting dozens of errors.
Table of Contents
* The Awful Truth about sed
* The essential command: s for substitution
* The slash as a delimiter
* Using & as the matched string
* Using \1 to keep part of the pattern
* Substitute Flags
* /g - Global replacement
* Is sed recursive?
* /1, /2, etc. Specifying which occurrence
* /p - print
* Write to a file with /w filename
* Combining substitution flags
* Arguments and invocation of sed
* Multiple commands with -e command
* Filenames on the command line
* sed -n: no printing
* sed -f scriptname
* sed in shell script
* Quoting multiple sed lines in the C shell
* Quoting multiple sed lines in the Bourne shell
* A sed interpreter script
* Sed Comments
* Passing arguments into a sed script
* Using sed in a shell here-is document
* Multiple commands and order of execution
* Addresses and Ranges of Text
* Restricting to a line number
* Patterns
* Ranges by line number
* Ranges by patterns
* Delete with d
* Printing with p
* Reversing the restriction with !
* Relationships between d, p, and !
* The q or quit command
* Grouping with { and }
* Writing a file with the 'w' command
* Reading in a file with the 'r' command
* SunOS and the # Comment Command
* Adding, Changing, Inserting new lines
* Append a line with 'a'
* Insert a line with 'i'
* Change a line with 'c'
* Leading tabs and spaces in a sed script
* Adding more than one line
* Adding lines and the pattern space
* Address ranges and the above commands
* Multi-Line Patterns
* Print line number with =
* Transform with y
* Displaying control characters with a l
* Working with Multiple Lines
* Using newlines in sed scripts
* The Hold Buffer
* Exchange with x
* Example of Context Grep
* Hold with h or H
* Keeping more than one line in the hold buffer
* Get with g or G
* Flow Control
* Testing with t
* An alternate way of adding comments
* The poorly undocumented ;
* Passing regular expressions as arguments
* Command Summary
* In Conclusion
* More References
Copyright 2001,2005,2007 Bruce Barnett and General Electric Company
All rights reserved
You are allowed to print copies of this tutorial for your personal
use, and link to this page, but you are not allowed to make electronic
copies, or redistribute this tutorial in any form without permission.
Introduction to Sed
How to use sed, a special editor for modifying files automatically. If
you want to write a program to make changes in a file, sed is the tool
to use.
There are a few programs that are the real workhorse in the Unix
toolbox. These programs are simple to use for simple applications, yet
have a rich set of commands for performing complex actions. Don't let
the complex potential of a program keep you from making use of the
simpler aspects. This chapter, like all of the rest, start with the
simple concepts and introduces the advanced topics later on.
A note on comments. When I first wrote this, most versions of sed did
not allow you to place comments inside the script. Lines starting with
the '#' characters are comments. Newer versions of sed may support
comments at the end of the line as well.
The Awful Truth about sed
Sed is the ultimate stream editor. If that sounds strange, picture a
stream flowing through a pipe. Okay, you can't see a stream if it's
inside a pipe. That's what I get for attempting a flowing analogy. You
want literature, read James Joyce.
Anyhow, sed is a marvelous utility. Unfortunately, most people never
learn its real power. The language is very simple, but the
documentation is terrible. The Solaris on-line manual pages for sed
are five pages long, and two of those pages describe the 34 different
errors you can get. A program that spends as much space documenting
the errors than it does documenting the language has a serious
learning curve.
Do not fret! It is not your fault you don't understand sed. I will
cover sed completely. But I will describe the features in the order
that I learned them. I didn't learn everything at once. You don't need
to either.
The essential command: s for substitution
Sed has several commands, but most people only learn the substitute
command: s. The substitute command changes all occurrences of the
regular expression into a new value. A simple example is changing
"day" in the "old" file to "night" in the "new" file:
sed s/day/night/ <old >new
Or another way (for Unix beginners),
sed s/day/night/ old >new
and for those who want to test this:
echo day | sed s/day/night/
This will output "night".
I didn't put quotes around the argument because this example didn't
need them. If you read my earlier tutorial, you would understand why
it doesn't need quotes. However, I recommend you do use quotes. If you
have meta-characters in the command, quotes are necessary. And if you
aren't sure, it's a good habit, and I will henceforth quote future
examples to emphasize the "best practice." Using the strong (single
quote) character, that would be:
sed 's/day/night/' <old >new
I must emphasize the the sed editor changes exactly what you tell it
to. So if you executed
echo Sunday | sed 's/day/night/' <old >new
This would output the word "Sunnight" bacause sed found the string
"day" in the input.
There are four parts to this substitute command:
s Substitute command
/../../ Delimiter
day Regular Expression Pattern Search Pattern
night Replacement string
The search pattern is on the left hand side and the replacement string
is on the right hand side.
We've covered quoting and regular expressions.. That's 90% of the
effort needed to learn the substitute command. To put it another way,
you already know how to handle 90% of the most frequent uses of sed.
There are a ... few fine points that an future sed expert should know
about. (You just finished section 1. There's only 63 more sections to
cover. :-) Oh. And you may want to bookmark this page, .... just in
case you don't finish.
The slash as a delimiter
The character after the s is the delimiter. It is conventionally a
slash, because this is what ed, more, and vi use. It can be anything
you want, however. If you want to change a pathname that contains a
slash - say /usr/local/bin to /common/bin - you could use the
backslash to quote the slash:
sed 's/\/usr\/local\/bin/\/common\/bin/' <old >new
Gulp. Some call this a 'Picket Fence' and it's ugly. It is easier to
read if you use an underline instead of a slash as a delimiter:
sed 's_/usr/local/bin_/common/bin_' <old >new
Some people use colons:
sed 's:/usr/local/bin:/common/bin:' <old >new
Others use the "|" character.
sed 's|/usr/local/bin|/common/bin|' <old >new
Pick one you like. As long as it's not in the string you are looking
for, anything goes. And remember that you need three delimiters. If
you get a "Unterminated `s' command" it's because you are missing one
of them.
Using & as the matched string
Sometimes you want to search for a pattern and add some characters,
like parenthesis, around or near the pattern you found. It is easy to
do this if you are looking for a particular string:
sed 's/abc/(abc)/' <old >new
This won't work if you don't know exactly what you will find. How can
you put the string you found in the replacement string if you don't
know what it is?
The solution requires the special character "&." It corresponds to the
pattern found.
sed 's/[a-z]*/(&)/' <old >new
You can have any number of "&" in the replacement string. You could
also double a pattern, e.g. the first number of a line:
% echo "123 abc" | sed 's/[0-9]*/& &/'
123 123 abc
Let me slightly amend this example. Sed will match the first string,
and make it as greedy as possible. The first match for '[0-9]*' is the
first character on the line, as this matches zero of more numbers. So
if the input was "abc 123" the output would be unchanged (well, except
for a space before the letters). A better way to duplicate the number
is to make sure it matches a number:
% echo "123 abc" | sed 's/[0-9][0-9]*/& &/'
123 123 abc
The string "abc" is unchanged, because it was not matched by the
regular expression. If you wanted to eliminate "abc" from the output,
you must expand the the regular expression to match the rest of the
line and explicitly exclude part of the expression using "(", ")" and
"\1", which is the next topic.
Using \1 to keep part of the pattern
I have already described the use of "(" ")" and "1" in my tutorial on
regular expressions.To review, the escaped parentheses (that is,
parentheses with backslashes before them) remember portions of the
regular expression. You can use this to exclude part of the regular
expression. The "\1" is the first remembered pattern, and the "\2" is
the second remembered pattern. Sed has up to nine remembered patterns.
If you wanted to keep the first word of a line, and delete the rest of
the line, mark the important part with the parenthesis:
sed 's/\([a-z]*\).*/\1/'
I should elaborate on this. Regular exprssions are greedy, and try to
match as much as possible. "[a-z]*" matches zero or more lower case
letters, and tries to be as big as possible. The ".*" matches zero or
more characters after the first match. Since the first one grabs all
of the lower case letters, the second matches anything else. Therefore
if you type
echo abcd123 | sed 's/\([a-z]*\).*/\1/'
This will output "abcd" and delete the numbers.
If you want to switch two words around, you can remember two patterns
and change the order around:
sed 's/\([a-z]*\) \([a-z]*\)/\2 \1/'
Note the space between the two remembered patterns. This is used to
make sure two words are found.
The "\1" doesn't have to be in the replacement string (in the right
hand side). It can be in the pattern you are searching for (in the
left hand side). If you want to eliminate duplicated words, you can
try:
sed 's/\([a-z]*\) \1/\1/'
You can have up to nine values: "\1" thru "\9."
Substitute Flags
You can add additional flags after the last delimiter. These flags can
specify what happens when there is more than one occurrence of a
pattern on a single line, and what to do if a substitution is found.
Let me describe them.
/g - Global replacement
Most Unix utilties work on files, reading a line at a time. Sed, by
default, is the same way. If you tell it to change a word, it will
only change the first occurrence of the word on a line. You may want
to make the change on every word on the line instead of the first. For
an example, let's place parentheses around words on a line. Instead of
using a pattern like "[A-Za-z]*" which won't match words like "won't,"
we will use a pattern, "[^ ]*," that matches everything except a
space. Well, this will also match anything because "*" means zero or
more. The current version of sed can get unhappy with patterns like
this, and generate errors like "Output line too long" or even run
forever. I consider this a bug, and have reported this to Sun. As a
work-around, you must avoid matching the null string when using the
"g" flag to sed. A work-around example is: "[^ ][^ ]*." The following
will put parenthesis around the first word:
sed 's/[^ ]*/(&)/' <old >new
If you want it to make changes for every word, add a "g" after the
last delimiter and use the work-around:
sed 's/[^ ][^ ]*/(&)/g' <old >new
Is sed recursive?
Sed only operates on patterns found in the in-coming data. That is,
the input line is read, and when a pattern is matched, the modified
output is generated, and the rest of the input line is scanned. The
"s" command will not scan the newly created output. That is, you don't
have to worry about expressions like:
sed 's/loop/loop the loop/g' <old >new
This will not cause an infinite loop. If a second "s" command is
executed, it could modify the results of a previous command. I will
show you how to execute multiple commands later.
/1, /2, etc. Specifying which occurrence
With no flags, the first pattern is changed. With the "g" option, all
patterns are changed. If you want to modify a particular pattern that
is not the first one on the line, you could use "\(" and "\)" to mark
each pattern, and use "\1" to put the first pattern back unchanged.
This next example keeps the first word on the line but deletes the
second:
sed 's/\([a-zA-Z]*\) \([a-zA-Z]*\) /\1 /' <old >new
Yuck. There is an easier way to do this. You can add a number after
the substitution command to indicate you only want to match that
particular pattern. Example:
sed 's/[a-zA-Z]* //2' <old >new
You can combine a number with the g (global) flag. For instance, if
you want to leave the first world alone alone, but change the second,
third, etc. to DELETED, use /2g:
sed 's/[a-zA-Z]* /DELETED /2g' <old >new
Don't get /2 and \2 confused. The /2 is used at the end. \2 is used in
inside the replacement field.
Note the space after the "*" character. Without the space, sed will
run a long, long time. (Note: this bug is probably fixed by now.) This
is because the number flag and the "g" flag have the same bug. You
should also be able to use the pattern
sed 's/[^ ]*//2' <old >new
but this also eats CPU. If this works on your computer, and it does on
some Unix systems, you could remove the encrypted password from the
password file:
sed 's/[^:]*//2' </etc/passwd >/etc/password.new
But this didn't work for me the time I wrote thise. Using "[^:][^:]*"
as a work-around doesn't help because it won't match an non-existent
password, and instead delete the third field, which is the user ID!
Instead you have to use the ugly parenthesis:
sed 's/^\([^:]*\):[^:]:/\1::/' </etc/passwd >/etc/password.new
You could also add a character to the first pattern so that it no
longer matches the null pattern:
sed 's/[^:]*:/:/2' </etc/passwd >/etc/password.new
The number flag is not restricted to a single digit. It can be any
number from 1 to 512. If you wanted to add a colon after the 80th
character in each line, you could type:
sed 's/./&:/80' <file >new
You can also do it the hard way by using 80 dots:
sed 's/^................................................................................/&:/'
<file >new
/p - print
By default, sed prints every line. If it makes a substitution, the new
text is printed instead of the old one. If you use an optional
argument to sed, "sed -n," it will not, by default, print any new
lines. I'll cover this and other options later. When the "-n" option
is used, the "p" flag will cause the modified line to be printed. Here
is one way to duplicate the function of grep with sed:
sed -n 's/pattern/&/p' <file
Write to a file with /w filename
There is one more flag that can follow the third delimiter. With it,
you can specify a file that will receive the modified data. An example
is the following, which will write all lines that start with an even
number to the file even:
sed -n 's/^[0-9]*[02468] /&/w even' <file
In this example, the output file isn't needed, as the input was not
modified. You must have exactly one space between the w and the
filename. You can also have ten files open with one instance of sed.
This allows you to split up a stream of data into separate files.
Using the previous example combined with multiple substitution
commands described later, you could split a file into ten pieces
depending on the last digit of the first number. You could also use
this method to log error or debugging information to a special file.
Combining substitution flags
You can combine flags when it makes sense. Also "w" has to be the last
flag. For example the following command works:
sed -n 's/a/A/2pw /tmp/file' <old >new
Next I will discuss the options to sed, and different ways to invoke sed.
Arguments and invocation of sed
previously, I have only used one substitute command. If you need to
make two changes, and you didn't want to read the manual, you could
pipe together multiple sed commands:
sed 's/BEGIN/begin/' <old | sed 's/END/end/' >new
This used two processes instead of one. A sed guru never uses two
processes when one can do.
Multiple commands with -e command
One method of combining multiple commands is to use a -e before each command:
sed -e 's/a/A/' -e 's/b/B/' <old >new
A "-e" isn't needed in the earlier examples because sed knows that
there must always be one command. If you give sed one argument, it
must be a command, and sed will edit the data read from standard
input.
Also see Quoting multiple sed lines in the Bourne shell
Filenames on the command line
You can specify files on the command line if you wish. If there is
more than one argument to sed that does not start with an option, it
must be a filename. This next example will count the number of lines
in three files that don't begin with a "#:"
sed 's/^#.*//' f1 f2 f3 | grep -v '^$' | wc -l
The sed substitute command changes every line that starts with a "#"
into a blank line. Grep was used to filter out empty lines. Wc counts
the number of lines left. Sed has more commands that make grep
unnecessary. But I will cover that later.
Of course you could write the last example using the "-e" option:
sed -e 's/^#.*//' f1 f2 f3 | grep -v '^$' | wc -l
There are two other options to sed.
sed -n: no printing
The "-n" option will not print anything unless an explicit request to
print is found. I mentioned the "/p" flag to the substitute command as
one way to turn printing back on. Let me clarify this. The command
sed 's/PATTERN/&/p' file
acts like the cat program if PATTERN is not in the file: e.g. nothing
is changed. If PATTERN is in the file, then each line that has this is
printed twice. Add the "-n" option and the example acts like grep:
sed -n 's/PATTERN/&/p' file
Nothing is printed, except those lines with PATTERN included.
sed -f scriptname
If you have a large number of sed commands, you can put them into a file and use
sed -f sedscript <old >new
where sedscript could look like this:
# sed comment - This script changes lower case vowels to upper case
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g
When there are several commands in one file, each command must be on a
separate line.
Also see here
sed in shell script
If you have many commands and they won't fit neatly on one line, you
can break up the line using a backslash:
sed -e 's/a/A/g' \
-e 's/e/E/g' \
-e 's/i/I/g' \
-e 's/o/O/g' \
-e 's/u/U/g' <old >new
Quoting multiple sed lines in the C shell
You can have a large, multi-line sed script in the C shell, but you
must tell the C shell that the quote is continued across several
lines. This is done by placing a backslash at the end of each line:
#!/bin/csh -f
sed 's/a/A/g \
s/e/E/g \
s/i/I/g \
s/o/O/g \
s/u/U/g' <old >new
Quoting multiple sed lines in the Bourne shell
The Bourne shell makes this easier as a quote can cover several lines:
#!/bin/sh
sed '
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g' <old >new
A sed interpreter script
Another way of executing sed is to use an interpreter script. Create a
file that contains:
#!/bin/sed -f
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g
Click here to get file: CapVowel.sed
If this script was stored in a file with the name "CapVowel" and was
executable, you could use it with the simple command:
CapVowel <old >new
Comments
Sed comments are lines where the first non-white character is a "#."
On many systems, sed can have only one comment, and it must be the
first line of the script. On the Sun (1988 when I wrote this), you can
have several comment lines anywhere in the script. Modern versions of
Sed support this. If the first line contains exactly "#n" then this
does the same thing as the "-n" option: turning off printing by
default. This could not done with a sed interpreter script, because
the first line must start with "#!/bin/sed -f" as I think "#!/bin/sed
-nf" generated an error. It works as I write this update in 2008. Note
that "#!/bin/sed -fn" does not work because sed thinks the filename of
the script is "n".
Passing arguments into a sed script
Passing a word into a shell script that calls sed is easy if you
remembered my tutorial on the Unix quoting mechanism. To review, you
use the single quotes to turn quoting on and off. A simple shell
script that uses sed to emulate grep is:
#!/bin/sh
sed -n 's/'$1'/&/p'
However - there is a problem with this script. If you have a space as
an argument, the script would cause a syntax error A better version
would protect from this happening:
#!/bin/sh
sed -n 's/'"$1"'/&/p'
Click here to get file: sedgrep.sed
If this was stored in a file called sedgrep, you could type
sedgrep '[A-Z][A-Z]' <file
This would allow sed to act as the grep command.
Using sed in a shell here-is document
You can use sed to prompt the user for some parameters and then create
a file with those parameters filled in. You could create a file with
dummy values placed inside it, and use sed to change those dummy
values. A simpler way is to use the "here is" document, which uses
part of the shell script as if it were standard input:
#!/bin/sh
echo -n 'what is the value? '
read value
sed 's/XXX/'$value'/' <<EOF
The value is XXX
EOF
Click here to get file: sed_hereis.sed
When executed, the script says:
what is the value?
If you type in "123," the next line will be:
The value is 123
I admit this is a contrived example. "Here is" documents can have
values evaluated without the use of sed. This example does the same
thing:
#!/bin/sh
echo -n 'what is the value? '
read value
cat <<EOF
The value is $value
EOF
However, combining "here is" documents with sed can be useful for some
complex cases.
Note that
sed 's/XXX/'$value'/' <<EOF
will give a syntax error if the user types a space. Better form would be to use
sed 's/XXX/'"$value"'/' <<EOF
Multiple commands and order of execution
As we explore more of the commands of sed, the commands will become
complex, and the actual sequence can be confusing. It's really quite
simple. Each line is read in. Each command, in order specified by the
user, has a chance to operate on the input line. After the
substitutions are made, the next command has a chance to operate on
the same line, which may have been modified by earlier commands. If
you ever have a question, the best way to learn what will happen is to
create a small example. If a complex command doesn't work, make it
simpler. If you are having problems getting a complex script working,
break it up into two smaller scripts and pipe the two scripts
together.
Addresses and Ranges of Text
You have only learned one command, and you can see how powerful sed
is. However, all it is doing is a grep and substitute. That is, the
substitute command is treating each line by itself, without caring
about nearby lines. What would be useful is the ability to restrict
the operation to certain lines. Some useful restrictions might be:
* Specifying a line by its number.
* Specifying a range of lines by number.
* All lines containing a pattern.
* All lines from the beginning of a file to a regular expression
* All lines from a regular expression to the end of the file.
* All lines between two regular expressions.
Sed can do all that and more. Every command in sed can be proceeded by
an address, range or restriction like the above examples. The
restriction or address immediately precedes the command:
restriction command
Restricting to a line number
The simplest restriction is a line number. If you wanted to delete the
first number on line 3, just add a "3" before the command:
sed '3 s/[0-9][0-9]*//' <file >new
Patterns
Many Unix utilities like vi and more use a slash to search for a
regular expression. Sed uses the same convention, provided you
terminate the expression with a slash. To delete the first number on
all lines that start with a "#," use:
sed '/^#/ s/[0-9][0-9]*//'
I placed a space after the "/expression/" so it is easier to read. It
isn't necessary, but without it the command is harder to fathom. Sed
does provide a few extra options when specifying regular expressions.
But I'll discuss those later. If the expression starts with a
backslash, the next character is the delimiter. To use a comma instead
of a slash, use:
sed '\,^#, s/[0-9][0-9]*//'
The main advantage of this feature is searching for slashes. Suppose
you wanted to search for the string "/usr/local/bin" and you wanted to
change it for "/common/all/bin." You could use the backslash to escape
the slash:
sed '/\/usr\/local\/bin/ s/\/usr\/local/\/common\/all/'
It would be easier to follow if you used an underline instead of a
slash as a search. This example uses the underline in both the search
command and the substitute command:
sed '\_/usr/local/bin_ s_/usr/local_/common/all_'
This illustrates why sed scripts get the reputation for obscurity. I
could be perverse and show you the example that will search for all
lines that start with a "g," and change each "g" on that line to an
"s:"
sed '/^g/s/g/s/g'
Adding a space and using an underscore after the substitute command
makes this much easier to read:
sed '/^g/ s_g_s_g'
Er, I take that back. It's hopeless. There is a lesson here: Use
comments liberally in a sed script under SunOS. You may have to remove
the comments to run the script under a different operating system, but
you now know how to write a sed script to do that very easily!
Comments are a Good Thing. You may have understood the script
perfectly when you wrote it. But six months from now it could look
like modem noise.
Ranges by line number
You can specify a range on line numbers by inserting a comma between
the numbers. To restrict a substitution to the first 100 lines, you
can use:
sed '1,100 s/A/a/'
If you know exactly how many lines are in a file, you can explicitly
state that number to perform the substitution on the rest of the file.
In this case, assume you used wc to find out there are 532 lines in
the file:
sed '101,532 s/A/a/'
An easier way is to use the special character "$," which means the
last line in the file.
sed '101,$ s/A/a/'
The "$" is one of those conventions that mean "last" in utilities like
cat -e, vi, and ed. "cat -e" Line numbers are cumulative if several
files are edited. That is,
sed '200,300 s/A/a/' f1 f2 f3 >new
is the same as
cat f1 f2 f3 | sed '200,300 s/A/a/' >new
Ranges by patterns
You can specify two regular expressions as the range. Assuming a "#"
starts a comment, you can search for a keyword, remove all comments
until you see the second keyword. In this case the two keywords are
"start" and "stop:"
sed '/start/,/stop/ s/#.*//'
The first pattern turns on a flag that tells sed to perform the
substitute command on every line. The second pattern turns off the
flag. If the "start" and "stop" pattern occurs twice, the substitution
is done both times. If the "stop" pattern is missing, the flag is
never turned off, and the substitution will be performed on every line
until the end of the file.
You should know that if the "start" pattern is found, the substitution
occurs on the same line that contains "start." This turns on a switch,
which is line oriented. That is, the next line is read and the
substitute command is checked. If it contains "stop" the switch is
turned off. Switches are line oriented, and not word oriented.
You can combine line numbers and regular expressions. This example
will remove comments from the beginning of the file until it finds the
keyword "start:"
sed -e '1,/start/ s/#.*//'
This example will remove comments everywhere except the lines between
the two keywords:
sed -e '1,/start/ s/#.*//' -e '/stop/,$ s/#.*//'
The last example has a range that overlaps the "/start/,/stop/" range,
as both ranges operate on the lines that contain the keywords. I will
show you later how to restrict a command up to, but not including the
line containing the specified pattern.
Before I start discussing the various commands, I should explain that
some commands cannot operate on a range of lines. I will let you know
when I mention the commands. In this next section I will describe
three commands, one of which cannot operate on a range.
Delete with d
Using ranges can be confusing, so you should expect to do some
experimentation when you are trying out a new script. A useful command
deletes every line that matches the restriction: "d." If you want to
look at the first 10 lines of a file, you can use:
sed '11,$ d' <file
which is similar in function to the head command. If you want to chop
off the header of a mail message, which is everything up to the first
blank line, use:
sed '1,/^$/ d' <file
You can duplicate the function of the tail command, assuming you know
the length of a file. Wc can count the lines, and expr can subtract 10
from the number of lines. A Bourne shell script to look at the last 10
lines of a file might look like this:
#!/bin/sh
#print last 10 lines of file
# First argument is the filename
lines=`wc -l $1 | awk '{print $1}' `
start=`expr $lines - 10`
sed "1,$start d" $1
Click here to get file: sed_tail.sh
The range for deletions can be regular expressions pairs to mark the
begin and end of the operation. Or it can be a single regular
expression. Deleting all lines that start with a "#" is easy:
sed '/^#/ d'
Removing comments and blank lines takes two commands. The first
removes every character from the "#" to the end of the line, and the
second deletes all blank lines:
sed -e 's/#.*//' -e '/^$/ d'
A third one should be added to remove all blanks and tabs immediately
before the end of line:
sed -e 's/#.*//' -e 's/[ ^I]*$//' -e '/^$/ d'
The character "^I" is a CRTL-I or tab character. You would have to
explicitly type in the tab. Note the order of operations above, which
is in that order for a very good reason. Comments might start in the
middle of a line, with white space characters before them. Therefore
comments are first removed from a line, potentially leaving white
space characters that were before the comment. The second command
removes all trailing blanks, so that lines that are now blank are
converted to empty lines. The last command deletes empty lines.
Together, the three commands remove all lines containing only
comments, tabs or spaces.
This demonstrates the pattern space sed uses to operate on a line. The
actual operation sed uses is:
* Copy the input line into the pattern space.
* Apply the first
sed command on the pattern space, if the address restriction is true.
* Repeat with the next sed expression, again
operating on the pattern space.
* When the last operation is performed, write out the pattern space
and read in the next line from the input file.
Printing with p
Another useful command is the print command: "p." If sed wasn't
started with an "-n" option, the "p" command will duplicate the input.
The command
sed 'p'
will duplicate every line. If you wanted to double every empty line, use:
sed '/^$/ p'
Adding the "-n" option turns off printing unless you request it.
Another way of duplicating head's functionality is to print only the
lines you want. This example prints the first 10 lines:
sed -n '1,10 p' <file
Sed can act like grep by combining the print operator to function on
all lines that match a regular expression:
sed -n '/match/ p'
which is the same as:
grep match
Reversing the restriction with !
Sometimes you need to perform an action on every line except those
that match a regular expression, or those outside of a range of
addresses. The "!" character, which often means not in Unix utilities,
inverts the address restriction. You remember that
sed -n '/match/ p'
acts like the grep command. The "-v" option to grep prints all lines
that don't contain the pattern. Sed can do this with
sed -n '/match/ !p' </tmp/b
Relationships between d, p, and !
As you may have noticed, there are often several ways to solve the
same problem with sed. This is because print and delete are opposite
functions, and it appears that "!p" is similar to "d," while "!d" is
similar to "p." I wanted to test this, so I created a 20 line file,
and tried every different combination. The following table, which
shows the results, demonstrates the difference:
Relations between d, p, and !
Sed Range Command Results
--------------------------------------------------------
sed -n 1,10 p Print first 10 lines
sed -n 11,$ !p Print first 10 lines
sed 1,10 !d Print first 10 lines
sed 11,$ d Print first 10 lines
--------------------------------------------------------
sed -n 1,10 !p Print last 10 lines
sed -n 11,$ p Print last 10 lines
sed 1,10 d Print last 10 lines
sed 11,$ !d Print last 10 lines
--------------------------------------------------------
sed -n 1,10 d Nothing printed
sed -n 1,10 !d Nothing printed
sed -n 11,$ d Nothing printed
sed -n 11,$ !d Nothing printed
--------------------------------------------------------
sed 1,10 p Print first 10 lines twice,
Then next 10 lines once
sed 11,$ !p Print first 10 lines twice,
Then last 10 lines once
--------------------------------------------------------
sed 1,10 !p Print first 10 lines once,
Then last 10 lines twice
sed 11,$ p Print first 10 lines once,
then last 10 lines twice
This table shows that the following commands are identical:
sed -n '1,10 p'
sed -n '11,$ !p'
sed '1,10 !d'
sed '11,$ d'
It also shows that the "!" command "inverts" the address range,
operating on the other lines.
The q or quit command
There is one more simple command that can restrict the changes to a
set of lines. It is the "q" command: quit. the third way to duplicate
the head command is:
sed '11 q'
which quits when the eleventh line is reached. This command is most
useful when you wish to abort the editing after some condition is
reached.
The "q" command is the one command that does not take a range of
addresses. Obviously the command
sed '1,10 q'
cannot quit 10 times. Instead
sed '1 q'
or
sed '10 q'
is correct.
Grouping with { and }
The curly braces, "{" and "}," are used to group the commands.
Hardly worth the build up. All that prose and the solution is just
matching squigqles. Well, there is one complication. Since each sed
command must start on its own line, the curly braces and the nested
sed commands must be on separate lines.
Previously, I showed you how to remove comments starting with a "#."
If you wanted to restrict the removal to lines between special "begin"
and "end" key words, you could use:
#!/bin/sh
# This is a Borne shell script that removes #-type comments
# between 'begin' and 'end' words.
sed -n '
/begin/,/end/ {
s/#.*//
s/[ ^I]*$//
/^$/ d
p
}
'
Click here to get file: sed_begin_end.sh
These braces can be nested, which allow you to combine address ranges.
You could perform the same action as before, but limit the change to
the first 100 lines:
#!/bin/sh
# This is a Borne shell script that removes #-type comments
# between 'begin' and 'end' words.
sed -n '
1,100 {
/begin/,/end/ {
s/#.*//
s/[ ^I]*$//
/^$/ d
p
}
}
'
Click here to get file: sed_begin_end1.sh
You can place a "!" before a set of curly braces. This inverts the
address, which removes comments from all lines except those between
the two reserved words:
#!/bin/sh
sed '
/begin/,/end/ !{
s/#.*//
s/[ ^I]*$//
/^$/ d
p
}
'
Click here to get file: sed_begin_end2.sh
Writing a file with the 'w' command
You may remember that the substitute command can write to a file:
sed -n 's/^[0-9]*[02468] /&/w even' <file
I used the "&" in the replacement part of the substitution command so
that the line would not be changed. A simpler example is to use the
"w" command, which has the same syntax as the "w" flag in the
substitute command:
sed -n '/^[0-9]*[02468]/ w even' <file
Remember - only one space must follow the command. Anything else will
be considered part of the file name. The "w" command also has the same
limitation as the "w" flag: only 10 files can be opened in sed.
Reading in a file with the 'r' command
There is also a command for reading files. The command
sed '$r end' <in>out
will append the file "end" at the end of the file (address "$)." The
following will insert a file after the line with the word "INCLUDE:"
sed '/INCLUDE/ r file' <in >out
You can use the curly braces to delete the line having the "INCLUDE"
command on it:
#!/bin/sh
sed '/INCLUDE/ {
r file
d
}'
Click here to get file: sed_include.sh
The order of the delete command "d" and the read file command "r" is
important. Change the order and it will not work. There are two subtle
actions that prevent this from working. The first is the "r" command
writes the file to the output stream. The file is not inserted into
the pattern space, and therefore cannot be modified by any command.
Therefore the delete command does not affect the data read from the
file.
The other subtlety is the "d" command deletes the current data in the
pattern space. Once all of the data is deleted, it does make sense
that no other action will be attempted. Therefore a "d" command
executed in a curly brace also aborts all further actions. As an
example, the substitute command below is never executed:
#!/bin/sh
# this example is WRONG
sed -e '1 {
d
s/.*//
}'
Click here to get file: sed_bad_example.sh
The earlier example is a crude version of the C preprocessor program.
The file that is included has a predetermined name. It would be nice
if sed allowed a variable (e.g "\1)" instead of a fixed file name.
Alas, sed doesn't have this ability. You could work around this
limitation by creating sed commands on the fly, or by using shell
quotes to pass variables into the sed script. Suppose you wanted to
create a command that would include a file like cpp, but the filename
is an argument to the script. An example of this script is:
% include 'sys/param.h' <file.c >file.c.new
A shell script to do this would be:
#!/bin/sh
# watch out for a '/' in the parameter
# use alternate search delimiter
sed -e '\_#INCLUDE <'"$1"'>_{
r '"$1"'
d
}'
Click here to get file: sed_include1.sh
SunOS and the # Comment Command
As we dig deeper into sed, comments will make the commands easier to
follow. Most versions of sed only allow one line as a comment, and it
must be the first line. SunOS allows more than one comment, and these
comments don't have to be first. The last example could be:
#!/bin/sh
# watch out for a '/' in the parameter
# use alternate search delimiter
sed -e '\_#INCLUDE <'"$1"'>_{
# read the file
r '"$1"'
# delete any characters in the pattern space
# and read the next line in
d
}'
Click here to get file: sed_include2.sh
Adding, Changing, Inserting new lines
Sed has three commands used to add new lines to the output stream.
Because an entire line is added, the new line is on a line by itself
to emphasize this. There is no option, an entire line is used, and it
must be on its own line. If you are familiar with many unix utilities,
you would expect sed to use a similar convention: lines are continued
by ending the previous line with a "\". The syntax to these commands
is finicky, like the "r" and "w" commands.
Append a line with 'a'
The "a" command appends a line after the range or pattern. This
example will add a line after every line with "WORD:"
#!/bin/sh
sed '
/WORD/ a\
Add this line after every line with WORD
'
Click here to get file: sed_add_line_after_word.sh
You could eliminate two lines in the shell script if you wish:
#!/bin/sh
sed '/WORD/ a\
Add this line after every line with WORD'
Click here to get file: sed_add_line_after_word1.sh
I prefer the first form because it's easier to add a new command by
adding a new line and because the intent is clearer. There must not be
a space after the "\".
Insert a line with 'i'
You can insert a new line before the pattern with the "i" command:
#!/bin/sh
sed '
/WORD/ i\
Add this line before every line with WORD
'
Click here to get file: sed_add_line_before_word.sh
Change a line with 'c'
You can change the current line with a new line.
#!/bin/sh
sed '
/WORD/ c\
Replace the current line with the line
'
Click here to get file: sed_change_line.sh
A "d" command followed by a "a" command won't work, as I discussed
earlier. The "d" command would terminate the current actions. You can
combine all three actions using curly braces:
#!/bin/sh
sed '
/WORD/ {
i\
Add this line before
a\
Add this line after
c\
Change the line to this one
}'
Click here to get file: sed_insert_append_change.sh
Leading tabs and spaces in a sed script
Sed ignores leading tabs and spaces in all commands. However these
white space characters may or may not be ignored if they start the
text following a "a," "c" or "i" command. In SunOS, both "features"
are available. The Berkeley (and Linux) style sed is in /usr/bin, and
the AT&T version (System V) is in /usr/5bin/.
To elaborate, the /usr/bin/sed command retains white space, while the
/usr/5bin/sed strips off leading spaces. If you want to keep leading
spaces, and not care about which version of sed you are using, put a
"\" as the first character of the line:
#!/bin/sh
sed '
a\
\ This line starts with a tab
'
Adding more than one line
All three commands will allow you to add more than one line. Just end
each line with a "\:"
#!/bin/sh
sed '
/WORD/ a\
Add this line\
This line\
And this line
'
Adding lines and the pattern space
I have mentioned the pattern space before. Most commands operate on
the pattern space, and subsequent commands may act on the results of
the last modification. The three previous commands, like the read file
command, add the new lines to the output stream, bypassing the pattern
space.
Address ranges and the above commands
You may remember in my last tutorial I warned you that some commands
can take a range of lines, and others cannot. To be precise, the
commands "a," "i," "r," and "q" will not take a range like "1,100" or
"/begin/,/end/." The documentation states that the read command can
take a range, but I get an error when I try this. The "c" or change
command allows this, and it will let you change several lines into
one:
#!/bin/sh
sed '
/begin/,/end/ c\
***DELETED***
'
If you need to do this, you can use the curly braces, as that will let
you perform the operation on every line:
#!/bin/sh
# add a blank line after every line
sed '1,$ {
a\
}'
Multi-Line Patterns
Most UNIX utilities are line oriented. Regular expressions are line
oriented. Searching for patterns that covers more than one line is not
an easy task. (Hint: It will be very shortly.)
Sed reads in a line of text, performs commands which may modify the
line, and outputs modification if desired. The main loop of a sed
script looks like this:
1. The next line is read from the input file and places in the
pattern space. If the end of file is found, and if there are
additional files to read, the current file is closed, the next file is
opened, and the first line of the new file is placed into the pattern
space.
2. The line count is incremented by one. Opening a new file does
not reset this number.
3. Each sed command is examined. If there is a restriction placed
on the command, and the current line in the pattern space meets that
restriction, the command is executed. Some commands, like "n" or "d"
cause sed to go to the top of the loop. The "q" command causes sed to
stop. Otherwise the next command is examined.
4. After all of the commands are examined, the pattern space is
output unless sed has the optional "-n" argument.
The restriction before the command determines if the command is
executed. If the restriction is a pattern, and the operation is the
delete command, then the following will delete all lines that have the
pattern:
/PATTERN/ d
If the restriction is a pair of numbers, then the deletion will happen
if the line number is equal to the first number or greater than the
first number and less than or equal to the last number:
10,20 d
If the restriction is a pair of patterns, there is a variable that is
kept for each of these pairs. If the variable is false and the first
pattern is found, the variable is made true. If the variable is true,
the command is executed. If the variable is true, and the last pattern
is on the line, after the command is executed the variable is turned
off:
/begin/,/end/ d
Whew! That was a mouthful. If you have read carefully up to here, you
should have breezed through this. You may want to refer back, because
I covered several subtle points. My choice of words was deliberate. It
covers some unusual cases, like:
# what happens if the second number
# is less than the first number?
sed -n '20,1 p' file
and
# generate a 10 line file with line numbers
# and see what happens when two patterns overlap
yes | head -10 | cat -n | \
sed -n -e '/1/,/7/ p' -e '/5/,/9/ p'
Enough mental punishment. Here is another review, this time in a table
format. Assume the input file contains the following lines:
AB
CD
EF
GH
IJ
When sed starts up, the first line is placed in the pattern space. The
next line line is "CD." The operations of the "n," "d," and "p"
commands can be summarized as:
+----------------+---------+------------------------------------------+
|Pattern Next | Command | Output New Pattern New Next |
|Space Input | | Space Input |
+----------------+---------+------------------------------------------+
|AB CD | n | <default> CD EF |
|AB CD | d | - CD EF |
|AB CD | p | AB CD EF |
+----------------+---------+------------------------------------------+
The "n" command may or may not generate output depending upon the
existence of the "-n" flag.
That review is a little easier to follow, isn't it? Before I jump into
multi-line patterns, I wanted to cover three more commands:
Print line number with =
The "=" command prints the current line number to standard output. One
way to find out the line numbers that contain a pattern is to use:
# add line numbers first,
# then use grep,
# then just print the number
cat -n file | grep 'PATTERN' | awk '{print $1}'
The sed solution is:
sed -n '/PATTERN/ =' file
Earlier I used the following to find the number of lines in a file
#!/bin/sh
lines=`wc -l file | awk '{print $1}' `
Using the "=" command can simplify this:
#!/bin/sh
lines=`sed -n '$=' file `
The "=" command only accepts one address, so if you want to print the
number for a range of lines, you must use the curly braces:
#!/bin/sh
# Just print the line numbers
sed -n '/begin/,/end/ {
=
d
}' file
Since the "=" command only prints to standard output, you cannot print
the line number on the same line as the pattern. You need to edit
multi-line patterns to do this.
Transform with y
If you wanted to change a word from lower case to upper case, you
could write 26 character substitutions, converting "a" to "A," etc.
Sed has a command that operates like the tr program. It is called the
"y" command. For instance, to change the letters "a" through "f" into
their upper case form, use:
sed 'y/abcdef/ABCDEF/' file
I could have used an example that converted all 26 letters into upper
case, and while this column covers a broad range of topics, the
"column" prefers a narrower format.
If you wanted to convert a line that contained a hexadecimal number
(e.g. 0x1aff) to upper case (0x1AFF), you could use:
sed '/0x[0-9a-zA-Z]*/ y/abcdef/ABCDEF' file
This works fine if there are only numbers in the file. If you wanted
to change the second word in a line to upper case, you are out of luck
- unless you use multi-line editing. (Hey - I think there is some sort
of theme here!)
Displaying control characters with a l
The "l" command prints the current pattern space. It is therefore
useful in debugging sed scripts. It also converts unprintable
characters into printing characters by outputting the value in octal
preceded by a "\" character. I found it useful to print out the
current pattern space, while probing the subtleties of sed.
Working with Multiple Lines
There are three new commands used in multiple-line patterns: "N," "D,"
and "P." I will explain their relation to the matching "n," "d," and
"p" single-line commands.
The "n" command will print out the current pattern space (unless the
"-n" flag is used), empty the current pattern space, and read in the
next line of input. The "N" command does not print out the current
pattern space and does not empty the pattern space. It reads in the
next line, but appends a new line character along with the input line
itself to the pattern space.
The "d" command deleted the current pattern space, reads in the next
line, puts the new line into the pattern space, and aborts the current
command, and starts execution at the first sed command. This is called
starting a new "cycle." The "D" command deletes the first portion of
the pattern space, up to the new line character, leaving the rest of
the pattern alone. Like "d," it stops the current command and starts
the command cycle over again. However, it will not print the current
pattern space. You must print it yourself, a step earlier. If the "D"
command is executed with a group of other commands in a curly brace,
commands after the "D" command are ignored. The next group of sed
commands is executed, unless the pattern space is emptied. If this
happens, the cycle is started from the top and a new line is read.
The "p" command prints the entire pattern space. The "P" command only
prints the first part of the pattern space, up to the NEWLINE
character.
Some examples might demonstrate "N" by itself isn't very useful. the filter
sed -e 'N'
doesn't modify the input stream. Instead, it combines the first and
second line, then prints them, combines the third and fourth line, and
prints them, etc. It does allow you to use a new "anchor" character:
"\n." This matches the new line character that separates multiple
lines in the pattern space. If you wanted to search for a line that
ended with the character "#," and append the next line to it, you
could use
#!/bin/sh
sed '
# look for a "#" at the end of the line
/#$/ {
# Found one - now read in the next line
N
# delete the "#" and the new line character,
s/#\n//
}' file
You could search for two lines containing "ONE" and "TWO" and only
print out the two consecutive lines:
#!/bin/sh
sed -n '
/ONE/ {
# found "ONE" - read in next line
N
# look for "TWO" on the second line
# and print if there.
/\n.*TWO/ p
}' file
The next example would delete everything between "ONE" and "TWO:"
#!/bin/sh
sed '
/ONE/ {
# append a line
N
# search for TWO on the second line
/\n.*TWO/ {
# found it - now edit making one line
s/ONE.*\n.*TWO/ONE TWO/
}
}' file
You can either search for a particular pattern on two consecutive
lines, or you can search for two consecutive words that may be split
on a line boundary. The next example will look for two words which are
either on the same line or one is on the end of a line and the second
is on the beginning of the next line. If found, the first word is
deleted:
#!/bin/sh
sed '
/ONE/ {
# append a line
N
# "ONE TWO" on same line
s/ONE TWO/TWO/
# "ONE
# TWO" on two consecutive lines
s/ONE\nTWO/TWO/
}
}' file
Let's use the
"D" command, and if we find a line containing
"TWO" immediately after a line containing
"ONE," then delete the first line:
#!/bin/sh
sed '
/ONE/ {
# append a line
N
# if TWO found, delete the first line
/\n.*TWO/ D
}' file
Click here to get file: sed_delete_line_after_word.sh
If we wanted to print the first line instead of deleting it, and not
print every other line, change the "D" to a "P" and add a "-n" as an
argument to sed:
#!/bin/sh
sed -n '
# by default - do not print anything
/ONE/ {
# append a line
N
# if TWO found, print the first line
/\n.*TWO/ P
}' file
Click here to get file: sed_print_line_after_word.sh
It is very common to combine all three multi-line commands. The
typical order is "N," "P" and lastly "D." This one will delete
everything between "ONE" and "TWO" if they are on one or two
consecutive lines:
#!/bin/sh
sed '
/ONE/ {
# append the next line
N
# look for "ONE" followed by "TWO"
/ONE.*TWO/ {
# delete everything between
s/ONE.*TWO/ONE TWO/
# print
P
# then delete the first line
D
}
}' file
Click here to get file: sed_delete_between_two_words.sh
Earlier I talked about the "=" command, and using it to add line
numbers to a file. You can use two invocations of sed to do this
(although it is possible to do it with one, but that must wait until
next section. The first sed command will output a line number on one
line, and then print the line on the next line. The second invocation
of sed will merge the two lines together:
#!/bin/sh
sed '=' file | \
sed '{
N
s/\n/ /
}'
Click here to get file: sed_merge_two_lines.sh
If you find it necessary, you can break one line into two lines, edit
them, and merge them together again. As an example, if you had a file
that had a hexadecimal number followed by a word, and you wanted to
convert the first word to all upper case, you can use the "y" command,
but you must first split the line into two lines, change one of the
two, and merge them together. That is, a line containing
0x1fff table2
will be changed into two lines:
0x1fff
table2
and the first line will be converted into upper case. I will use tr to
convert the space into a new line:
#!/bin/sh
tr ' ' '\012' file|
sed ' {
y/abcdef/ABCDEF/
N
s/\n/ /
}'
Click here to get file: sed_split.sh
It isn't obvious, but sed could be used instead of tr. You can embed a
new line in a substitute command, but you must escape it with a
backslash. It is unfortunate that you must use "\n" in the left side
of a substitute command, and an embedded new line in the right hand
side. Heavy sigh. Here is the example:
#!/bin/sh
sed '
s/ /\
/' | \
sed ' {
y/abcdef/ABCDEF/
N
s/\n/ /
}'
Click here to get file: sed_split_merge.sh
Sometimes I add a special character as a marker, and look for that
character in the input stream. When found, it indicates the place a
blank used to be. A backslash is a good character, except it must be
escaped with a backslash, and makes the sed script obscure. Save it
for that guy who keeps asking dumb questions. The sed script to change
a blank into a "\" following by a new line would be:
#!/bin/sh
sed 's/ /\\\
/' file
Click here to get file: sed_addslash_before_blank.sh
Yeah. That's the ticket. Or use the C shell and really confuse him!
#!/bin/csh -f
sed '\
s/ /\\\\
/' file
Click here to get file: sed_addslash_before_blank.csh
A few more examples of that, and he'll never ask you a question again!
I think I'm getting carried away. I'll summarize with a chart that
covers the features we've talked about:
+----------------+---------+------------------------------------------+
|Pattern Next | Command | Output New Pattern New Next |
|Space Input | | Space Input |
+----------------+---------+------------------------------------------+
|AB CD | n | <default> CD EF |
|AB CD | N | - AB\nCD EF |
|AB CD | d | - CD EF |
|AB CD | D | - CD EF |
|AB CD | p | AB CD EF |
|AB CD | P | AB CD EF |
+----------------+---------+------------------------------------------+
|AB\nCD EF | n | <default> EF GH |
|AB\nCD EF | N | - AB\nCD\nEF GH |
|AB\nCD EF | d | - EF GH |
|AB\nCD EF | D | - CD EF |
|AB\nCD EF | p | AB\nCD AB\nCD EF |
|AB\nCD EF | P | AB AB\nCD EF |
+----------------+---------+------------------------------------------+
Using newlines in sed scripts
Occasionally one wishes to use a new line character in a sed script.
Well, this has some subtle issues here. If one wants to search for a
new line, one has to use "\n." Here is an example where you search for
a phrase, and delete the new line character after that phrase -
joining two lines together.
(echo a;echo x;echo y) | sed '/x$/ {
N
s:x\n:x:
}'
which generates
a
xy
However, if you are inserting a new line, don't use "\n" - instead
insert a literal new line character:
(echo a;echo x;echo y) | sed 's:x:X\
:'
generates
a
X
y
The Hold Buffer
So far we have talked about three concepts of sed: (1) The input
stream or data before it is modified, (2) the output stream or data
after it has been modified, and (3) the pattern space, or buffer
containing characters that can be modified and send to the output
stream.
There is one more "location" to be covered: the hold buffer or hold
space. Think of it as a spare pattern buffer. It can be used to "copy"
or "remember" the data in the pattern space for later. There are five
commands that use the hold buffer.
Exchange with x
The "x" command eXchanges the pattern space with the hold buffer. By
itself, the command isn't useful. Executing the sed command
sed 'x'
as a filter adds a blank line in the front, and deletes the last line.
It looks like it didn't change the input stream significantly, but the
sed command is modifying every line.
The hold buffer starts out containing a blank line. When the "x"
command modifies the first line, line 1 is saved in the hold buffer,
and the blank line takes the place of the first line. The second "x"
command exchanges the second line with the hold buffer, which contains
the first line. Each subsequent line is exchanged with the preceding
line. The last line is placed in the hold buffer, and is not exchanged
a second time, so it remains in the hold buffer when the program
terminates, and never gets printed. This illustrates that care must be
taken when storing data in the hold buffer, because it won't be output
unless you explicitly request it.
Example of Context Grep
One use of the hold buffer is to remember previous lines. An example
of this is a utility that acts like grep as it shows you the lines
that match a pattern. In addition, it shows you the line before and
after the pattern. That is, if line 8 contains the pattern, this
utility would print lines 7, 8 and 9.
One way to do this is to see if the line has the pattern. If it does
not have the pattern, put the current line in the hold buffer. If it
does, print the line in the hold buffer, then the current line, and
then the next line. After each set, three dashes are printed. The
script checks for the existence of an argument, and if missing, prints
an error. Passing the argument into the sed script is done by turning
off the single quote mechanism, inserting the "$1" into the script,
and starting up the single quote again:
#!/bin/sh
# grep3 - prints out three lines around pattern
# if there is only one argument, exit
case $# in
1);;
*) echo "Usage: $0 pattern";exit;;
esac;
# I hope the argument doesn't contain a /
# if it does, sed will complain
# use sed -n to disable printing
# unless we ask for it
sed -n '
'/$1/' !{
#no match - put the current line in the hold buffer
x
# delete the old one, which is
# now in the pattern buffer
d
}
'/$1/' {
# a match - get last line
x
# print it
p
# get the original line back
x
# print it
p
# get the next line
n
# print it
p
# now add three dashes as a marker
a\
---
# now put this line into the hold buffer
x
}'
Click here to get file: grep3.sh
You could use this to show the three lines around a keyword, i.e.:
grep3 vt100 </etc/termcap
Hold with h or H
The "x" command exchanges the hold buffer and the pattern buffer. Both
are changed. The "h" command copies the pattern buffer into the hold
buffer. The pattern buffer is unchanged. An identical script to the
above uses the hold commands:
#!/bin/sh
# grep3 version b - another version using the hold commands
# if there is only one argument, exit
case $# in
1);;
*) echo "Usage: $0 pattern";exit;;
esac;
# again - I hope the argument doesn't contain a /
# use sed -n to disable printing
sed -n '
'/$1/' !{
# put the non-matching line in the hold buffer
h
}
'/$1/' {
# found a line that matches
# append it to the hold buffer
H
# the hold buffer contains 2 lines
# get the next line
n
# and add it to the hold buffer
H
# now print it back to the pattern space
x
# and print it.
p
# add the three hyphens as a marker
a\
---
}'
Click here to get file: grep3a.sh
Keeping more than one line in the hold buffer
The "H" command allows you to combine several lines in the hold
buffer. It acts like the "N" command as lines are appended to the
buffer, with a "\n" between the lines. You can save several lines in
the hold buffer, and print them only if a particular pattern is found
later.
As an example, take a file that uses spaces as the first character of
a line as a continuation character. The files /etc/termcap,
/etc/printcap, makefile and mail messages use spaces or tabs to
indicate a continuing of an entry. If you wanted to print the entry
before a word, you could use this script. I use a "^I" to indicate an
actual tab character:
#!/bin/sh
# print previous entry
sed -n '
/^[ ^I]/!{
# line does not start with a space or tab,
# does it have the pattern we are interested in?
'/$1/' {
# yes it does. print three dashes
i\
---
# get hold buffer, save current line
x
# now print what was in the hold buffer
p
# get the original line back
x
}
# store it in the hold buffer
h
}
# what about lines that start
# with a space or tab?
/^[ ^I]/ {
# append it to the hold buffer
H
}'
Click here to get file: grep_previous.sh
You can also use the "H" to extend the context grep. In this example,
the program prints out the two lines before the pattern, instead of a
single line. The method to limit this to two lines is to use the "s"
command to keep one new line, and deleting extra lines. I call it
grep4:
#!/bin/sh
# grep4: prints out 4 lines around pattern
# if there is only one argument, exit
case $# in
1);;
*) echo "Usage: $0 pattern";exit;;
esac;
sed -n '
'/$1/' !{
# does not match - add this line to the hold space
H
# bring it back into the pattern space
x
# Two lines would look like .*\n.*
# Three lines look like .*\n.*\n.*
# Delete extra lines - keep two
s/^.*\n\(.*\n.*\)$/\1/
# now put the two lines (at most) into
# the hold buffer again
x
}
'/$1/' {
# matches - append the current line
H
# get the next line
n
# append that one also
H
# bring it back, but keep the current line in
# the hold buffer. This is the line after the pattern,
# and we want to place it in hold in case the next line
# has the desired pattern
x
# print the 4 lines
p
# add the mark
a\
---
}'
Click here to get file: grep4.sh
You can modify this to print any number of lines around a pattern. As
you can see, you must remember what is in the hold space, and what is
in the pattern space. There are other ways to write the same routine.
Get with g or G
Instead of exchanging the hold space with the pattern space, you can
copy the hold space to the pattern space with the "g" command. This
deletes the pattern space. If you want to append to the pattern space,
use the "G" command. This adds a new line to the pattern space, and
copies the hold space after the new line.
Here is another version of the "grep3" command. It works just like the
previous one, but is implemented differently. This illustrates that
sed has more than one way to solve many problems. What is important is
you understand your problem, and document your solution:
#!/bin/sh
# grep3 version c: use 'G' instead of H
# if there is only one argument, exit
case $# in
1);;
*) echo "Usage: $0 pattern";exit;;
esac;
# again - I hope the argument doesn't contain a /
sed -n '
'/$1/' !{
# put the non-matching line in the hold buffer
h
}
'/$1/' {
# found a line that matches
# add the next line to the pattern space
N
# exchange the previous line with the
# 2 in pattern space
x
# now add the two lines back
G
# and print it.
p
# add the three hyphens as a marker
a\
---
# remove first 2 lines
s/.*\n.*\n\(.*\)$/\1/
# and place in the hold buffer for next time
h
}'
Click here to get file: grep3c.sh
The "G" command makes it easy to have two copies of a line. Suppose
you wanted to the convert the first hexadecimal number to uppercase,
and don't want to use the script I described in an earlier column
#!/bin/sh
# change the first hex number to upper case format
# uses sed twice
# used as a filter
# convert2uc <in >out
sed '
s/ /\
/' | \
sed ' {
y/abcdef/ABCDEF/
N
s/\n/ /
}'
Click here to get file: convert2uc.sh
Here is a solution that does not require two invocations of sed:
#!/bin/sh
# convert2uc version b
# change the first hex number to upper case format
# uses sed once
# used as a filter
# convert2uc <in >out
sed '
{
# remember the line
h
#change the current line to upper case
y/abcdef/ABCDEF/
# add the old line back
G
# Keep the first word of the first line,
# and second word of the second line
# with one humongeous regular expression
s/^\([^ ]*\) .*\n[^ ]* \(.*\)/\1 \2/
}'
Click here to get file: convert2uc1.sh
Carl Henrik Lunde suggested a way to make this simpler. I was working too hard.
#!/bin/sh
# convert2uc version b
# change the first hex number to upper case format
# uses sed once
# used as a filter
# convert2uc <in >out
sed '
{
# remember the line
h
#change the current line to upper case
y/abcdef/ABCDEF/
# add the old line back
G
# Keep the first word of the first line,
# and second word of the second line
# with one humongeous regular expression
s/ .* / / # delete all but the first and last word
}'
Click here to get file: convert2uc2.sh
This example only converts the letters "a" through "f" to upper case.
This was chosen to make the script easier to print in these narrow
columns. You can easily modify the script to convert all letters to
uppercase, or to change the first letter, second word, etc.
Flow Control
As you learn about sed you realize that it has it's own programming
language. It is true that it's a very specialized and simple language.
What language would be complete without a method of changing the flow
control? There are three commands sed uses for this. You can specify a
label with an text string followed by a colon. The "b" command
branches to the label. The label follows the command. If no label is
there, branch to the end of the script. The "t" command is used to
test conditions. Before I discuss the "t" command, I will show you an
example using the "b" command.
This example remembers paragraphs, and if it contains the pattern
(specified by an argument), the script prints out the entire
paragraph.
#!/bin/sh
sed -n '
# if an empty line, check the paragraph
/^$/ b para
# else add it to the hold buffer
H
# at end of file, check paragraph
$ b para
# now branch to end of script
b
# this is where a paragraph is checked for the pattern
:para
# return the entire paragraph
# into the pattern space
x
# look for the pattern, if there - print
/'$1'/ p
'
Click here to get file: grep_paragraph.sh
Testing with t
You can execute a branch if a pattern is found. You may want to
execute a branch only if a substitution is made. The command "t label"
will branch to the label if the last substitute command modified the
pattern space.
One use for this is recursive patterns. Suppose you wanted to remove
white space inside parenthesis. These parentheses might be nested.
That is, you would want to delete a string that looked like "( ( (
())) )." The sed expressions
sed 's/([ ^I]*)/g'
would only remove the innermost set. You would have to pipe the data
through the script four times to remove each set or parenthesis. You
could use the regular expression
sed 's/([ ^I()]*)/g'
but that would delete non-matching sets of parenthesis. The "t"
command would solve this:
#!/bin/sh
sed '
:again
s/([ ^I]*)//g
t again
'
Click here to get file: delete_nested_parens.sh
An alternate way of adding comments
There is one way to add comments in a sed script if you don't have a
version that supports it. Use the "a" command with the line number of
zero:
#!/bin/sh
sed '
/begin/ {
0i\
This is a comment\
It can cover several lines\
It will work with any version of sed
}'
Click here to get file: sed_add_comments.sh
The poorly undocumented ;
There is one more sed command that isn't well documented. It is the
";" command. This can be used to combined several sed commands on one
line. Here is the grep4 script I described earlier, but without the
comments or error checking and with semicolons between commands:
#!/bin/sh
sed -n '
'/$1/' !{;H;x;s/^.*\n\(.*\n.*\)$/\1/;x;}
'/$1/' {;H;n;H;x;p;a\
---
}'
Click here to get file: grep4a.sh
Yessireebob! Definitely character building. I think I have made my
point. As far as I am concerned, the only time the semicolon is useful
is when you want to type the sed script on the command line. If you
are going to place it in a script, format it so it is readable. I have
mentioned earlier that many versions of sed do not support comments
except on the first line. You may want to write your scripts with
comments in them, and install them in "binary" form without comments.
This should not be difficult. After all, you have become a sed guru by
now. I won't even tell you how to write a script to strip out
comments. That would be insulting your intelligence.
Passing regular expressions as arguments
In the earlier scripts, I mentioned that you would have problems if
you passed an argument to the script that had a slash in it. In fact,
regular expression might cause you problems. A script like the
following is asking to be broken some day:
#!/bin/sh
sed 's/'"$1"'//g'
If the argument contains any of these characters in it, you may get a
broken script: "/\.*[]^$" For instance, if someone types a "/" then
the substiture command will see four delimiters instead of three. You
will also get syntax errors if you provide a "]" without a "]". One
solution is to have the user put a backslash before any of these
characters when they pass it as an argument. However, the user has to
know which characters are special.
Another solution is to add a backslash before each of those characters
in the script
#!/bin/sh
arg=`echo "$1" | sed 's:[]\[\^\$\.\*\/]:\\\\&:g'`
sed 's/'"$arg"'//g'
Click here to get file: sed_with_regular_expressions1.sh
If you were searching for the pattern "^../," the script would convert
this into "\^\.\.\/" before passing it to sed.
Command Summary
As I promised earlier, here is a table that summarizes the different
commands. The second column specifies if the command can have a range
or pair of addresses (with a 2) or a single address or pattern (with a
1). The next four columns specifies which of the four buffers or
streams are modified by the command. Some commands only affect the
output stream, others only affect the hold buffer. If you remember
that the pattern space is output (unless a "-n" was given to sed),
this table should help you keep track of the various commands.
+--------------------------------------------------------+
|Command Address Modifications to |
| or Range Input Output Pattern Hold |
| Stream Stream Space Buffer |
+--------------------------------------------------------+
|= - - Y - - |
|a 1 - Y - - |
|b 2 - - - - |
|c 2 - Y - - |
|d 2 Y - Y - |
|D 2 Y - Y - |
|g 2 - - Y - |
|G 2 - - Y - |
|h 2 - - - Y |
|H 2 - - - Y |
|i 1 - Y - - |
|l 1 - Y - - |
|n 2 Y * - - |
|N 2 Y - Y - |
|p 2 - Y - - |
|P 2 - Y - - |
|q 1 - - - - |
|r 1 - Y - - |
|s 2 - - Y - |
|t 2 - - - - |
|w 2 - Y - - |
|x 2 - - Y Y |
|y 2 - - Y - |
+--------------------------------------------------------+
The "n" command may or may not generate output, depending on the "-n"
option. The "r" command can only have one address, despite the
documentation.
In Conclusion
This concludes my tutorial on sed. It is possible to find shorter
forms of some of my scripts. However, I chose these examples to
illustrate some basic constructs. I wanted clarity, not obscurity. I
hope you enjoyed it.
More References
This concludes my tutorial on Other of my Unix shell tutorials can be
found here. Other shell tutorials can be found at Heiner's SHELLdorado
and Chris F. A. Johnson's Unix Shell Page The Wikipedia Entry on SED
And don't forget The SED FAQ
This document was originally converted from NROFF to TEXT to HTML.
please forgive errors in the translation.
If you are confused, grab the actual script if possible. No
translations occured in the scripts.
Thanks to Carl Henrik Lunde who suggested an improvement to
convert2uc1.sh This document was translated by troff2html v0.21 on
September 22, 2001 and then manually edited to make it compliant with:
Valid HTML 4.01!
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