[Bug 1919185] [NEW] Bash completion breaks the content of $_ variable

[libcha]

Public bug reported:

In bash, the $_ variable ought to contain the last parameter of previous
command. For example:

$ touch aaa bbb
$ cat aaa
$ cat bbb $_

the last line is equivalent to "cat bbb aaa".

However, if I use bash completion to fill some other parameter like
this:

$ cat bb<TAB> $_

(where <TAB> denotes hitting the TAB key), the value of the $_ variable
is overwritten with some mess, so that the result is: "cat: _filedir: No
such file or directory"

I expect this variable not to be overwritten and still contain the value
"aaa", co that the command prints out the files bbb and aaa.

This bug seems to be version-wide and usage-wide. I encounter this for
many Ubuntu releases, for example, using Kubuntu 20.10. It can be
reproduced even in terminal with no Xorg running.  The resulting error
message differs according to what is being TAB-completed, for example
with git command it says: "error: pathspec '__git_main' did not match
any file(s) known to git".

** Affects: bash (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1919185

Title:
  Bash completion breaks the content of $_ variable

Status in bash package in Ubuntu:
  New

Bug description:
  In bash, the $_ variable ought to contain the last parameter of
  previous command. For example:

  $ touch aaa bbb
  $ cat aaa
  $ cat bbb $_

  the last line is equivalent to "cat bbb aaa".

  However, if I use bash completion to fill some other parameter like
  this:

  $ cat bb<TAB> $_

  (where <TAB> denotes hitting the TAB key), the value of the $_
  variable is overwritten with some mess, so that the result is: "cat:
  _filedir: No such file or directory"

  I expect this variable not to be overwritten and still contain the
  value "aaa", co that the command prints out the files bbb and aaa.

  This bug seems to be version-wide and usage-wide. I encounter this for
  many Ubuntu releases, for example, using Kubuntu 20.10. It can be
  reproduced even in terminal with no Xorg running.  The resulting error
  message differs according to what is being TAB-completed, for example
  with git command it says: "error: pathspec '__git_main' did not match
  any file(s) known to git".

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