Diff for "BashFAQ/002"

How can I store the return value/output of a command in a variable?

Well, that depends on exactly what you mean by that question. Some people want to store the command's output (either stdout, or stdout + stderr); and others want to store the command's exit status (0 to 255, with 0 typically meaning "success").

If you want to capture the output, you use command substitution:

    var=$(command)      # stdout only; stderr remains uncaptured
    var=$(command 2>&1) # both stdout and stderr will be captured

If you want the exit status:

    command
    var=$?

If you want both:

    var1=$(command)
    var2=$?

The assignment to var1 has no effect on command's exit status, which is still in $?.

If you don't actually want the exit status, but simply want to take an action upon success or failure:

    if command
    then
        echo "it succeeded"
    else
        echo "it failed"
    fi

Or (shorter):

    command && echo "it succeeded" || echo "it failed"

What if you want the exit status of a command in a few that are piped to each other? Use the PIPESTATUS array (BASH only). Say you want the exit status of grep in the following:

    grep foo somelogfile | head -5
    result=${PIPESTATUS[0]}

Now, some trickier stuff. Let's say you want only the stderr, but not stdout. Well, then first you have to decide where you do want stdout to go:

    var=$(command 2>&1 >/dev/null)  # Save stderr, discard stdout.
    var=$(command 2>&1 >/dev/tty)   # Save stderr, send stdout to the terminal.
    var=$(command 3>&2 2>&1 1>&3-)  # Save stderr, send stdout to stderr

It's possible, although considerably harder, to let stdout "fall through" to wherever it would've gone if there hadn't been any redirection. This involves "saving" the current value of stdout, so that it can be used inside the command substitution:

    exec 3>&1                    # Save the place that stdout (1) points to.
    var=$(command 2>&1 1>&3)     # Run command.  stderr is captured.
    exec 3>&-                    # Close FD #3.

    # Or this alternative:
    { var=$(command 2>&1 1>&3-) ;} 3>&1 # Capture stderr, let stdout through.

In the last example above, note that 1>&3- duplicates FD 3 and stores a copy in FD 1, and then closes FD 3.

What you cannot do is capture stdout in one variable, and stderr in another, using only FD redirections. You must use a temporary file to achieve that one.

Well, you can use a horrible hack like:

   result=$( { stdout=$(cmd) ; } 2>&1; echo "this line is the separator"; echo "$stdout")
   var_out=${result#*this line is the separator$'\n'}
   var_err=${result%$'\n'this line is the separator*}

Obviously, this is not robust, because either the standard output or the standard error of the command could contain whatever separator string you employ.