Reading a variable line by line with while loop

[James Michael Fultz]

* Ray Parrish <crp at cmc.net> [2009-12-01 15:29 -0800]:
> Well, I'm making some progress. I have the following code which works to 
> a point.
> 
>      while read ThisCommand; do
>            ThisCommand=${ThisCommand// /__};
>            if `echo "${History}" | grep "${ThisCommand}" 1>/dev/null 2>&1`
>                then
>                    echo "nothing" >/dev/null

The colon (:) builtin which evaluates as a successful command execution
is useful where you to effectively do nothing in your script.

: does nothing

You could also perform a negative test in order to avoid the do-nothing
block of your if-statement.  Also, you can evaluate grep's exit status
directly rather than the output of the command.  You also probably want
to test for fixed strings rather than regexes, thus the '-F' option to
grep.

if ! echo "$History" | grep -F "$ThisCommand" >/dev/null

>                else
>                    History="$History $ThisCommand";
>            fi
>      done < <(cat ~/.bash_history)

Here, you don't need cat (nor process substitution) at all and could
write it more efficiently.

done < ~/.bash_history

Also, if you're using Bash and want to read the contents of a file into
a variable, you can spare using cat as well.

FOO=`cat foobar.txt`

or

FOO=$(cat foobar.txt)

becomes

FOO=$(< foobar.txt)

>      Command=`Xdialog --stdout --title "Ray's Bash Manager - Select 
> Command to Copy" --cancel-label "Exit" --combobox "Select a Command to 
> Copy to the Clipboard." 0 60 $History`
> 
> The if statement is meant to catch duplicate lines, and it does, unless 
> they contain quotes in them, and then they get duplicated in the output. 
> Is there some way to trap even the lines with quotes in them?

Appears the conditional in your if-statement was faulty.  if itself acts
only upon the exit status of the command list given to it -- not the
output of those commands.

if [ "$FOO" = "xyz" ]

It's actually the [ command performing the expression evaluation.

if [[ "$FOO" == "xyz" ]]

Again, the [[ command evaluates the expression.

if echo "$FOO" | grep xyz

Here, a pipeline using grep is used to test for the presence of
a string.