Re: [man bash] help understanding.

From: Stephen Bach (sjbach_at_users.sourceforge.net)
Date: 04/27/05 

Date: Wed, 27 Apr 2005 14:02:02 -0400

Hi nabis,

nabis wrote:
> Hi!
> I need some additional explanation on section PARAMETERS subsection
> Special Parameters from the bash(1) manual page. I states:
>
> _ At shell startup, set to the absolute file name of the shell or
> shell script being executed as passed in the argument list. Subse-
> quently, expands to the last argument to the previous command,
> after expansion. Also set to the full file name of each command
> executed and placed in the environment exported to that command.
> When checking mail, this parameter holds the name of the mail file
> currently being checked.
>
> If you translate it in code it will, probably, look like the following:
>
> #!/usr/bin/bash
> echo $_ # the absolute file name of the shell
> ls /home
> echo $_ # the last argument to the previous command
>
> I'd appreciate if someone could explain better or type in a line of code
> which demonstrates the following:
> "Also set to the full file name of each command executed and placed in
> the environment exported to that command."
>
> Thank you. Vitaliy.

It just means that the new process will see the $_ variable as being
the path to its executable (not as the last argument to the
command called previous to it).

For example, given a script and a short program:

foo.sh:
        #!/bin/bash
        echo $_
        ls /
        echo $_

        ./bar abc def ghi

        echo $_

bar.c:
        #include <unistd.h>
        #include <stdlib.h>
        #include <stdio.h>

        int main(int argc, char** argv) {

                printf("%s\n", getenv("_"));

                return 0;
        }

The output from running ./foo.sh:

        /bin/bash
        bin dev home lost+found opt root sys usr
        boot etc lib mnt proc sbin tmp var
        /
        ./bar
        ghi

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