BASH - Output - Get the output of a command

This depends on whether you want to store the command's output (either stdout, or stdout + stderr) or its exit status (0 to 255, with 0 typically meaning “success”).

Capture the output of a command

To capture the output, you use command substitution:

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

To save both output of a command; and the exit status

If you want both:

output=$(command)
status=$?

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

Capture stdout and take action on exit status

If you want to capture stdout as well as taking action on success/failure, without explicitly storing or checking $?:

if output=$(command); then
    printf "it succeeded\n"
    ...

Capture stdout and stderr

What you cannot do is capture stdout in one variable, and stderr in another, using only FD redirections.

You must use a temporary file (or a named pipe) to achieve that one.

Well, you can use a horrible hack like:

cmd() { curl -s -v http://www.google.fr; }
 
result=$(
    { stdout=$(cmd) ; } 2>&1
    printf "this line is the separator\n"
    printf "%s\n" "$stdout"
)
var_out=${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.

And if you want the exit code of your cmd (here a modification in the case of if the cmd stdout nothing)

cmd() { curl -s -v http://www.google.fr; }
 
result=$(
    { stdout=$(cmd); returncode=$?; } 2>&1
    printf "this is the separator"
    printf "%s\n" "$stdout"
    exit "$returncode"
)
returncode=$?
 
var_out=${result#*this is the separator}
var_err=${result%this is the separator*}

Only keep stderr

Say you want only the stderr, but not stdout.

Then first you have to decide where you do want stdout to go:

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

Since the last example may seem a bit confusing, here is the explanation.

First, keep in mind that 1>&3- is equivalent to 1>&3 3>&-.
So it will be easier to analyze the following sequence: $(… 3>&2 2>&1 1>&3 3>&-)

Redirection	fd 0 (stdin)	fd 1 (stdout)	fd 2 (stderr)	fd 3	Description
initial	/dev/tty	/dev/tty	/dev/tty		Let's assume this is run in a terminal, so stdin, stdout and stderr are all initially connected to the terminal (tty).
$(…)	/dev/tty	pipe	/dev/tty		First, the command substitution is set up. Command's stdout (FileDescriptor 1) gets captured (by using a pipe internally). Command's stderr (FD 2) still points to its regular place (the script's stderr).
3>&2	/dev/tty	pipe	/dev/tty	/dev/tty	Next, FD 3 should point to what FD 2 points to at this very moment, meaning FD 3 will point to the script's stderr (“save stderr in FD 3”).
2>&1	/dev/tty	pipe	pipe	/dev/tty	Next, FD 2 should point to what FD 1 currently points to, meaning FD 2 will point to stdout. Right now, both FD 2 and FD 1 would be captured.
1>&3	/dev/tty	/dev/tty	pipe	/dev/tty	Next, FD 1 should point to what FD 3 currently points to, meaning FD 1 will point to the script's stderr. FD 1 is no longer captured. We have “swapped” FD 1 and FD 2.
3>&-	/dev/tty	/dev/tty	pipe		Finally, we close FD 3 as it is no longer necessary.

A little note: operation n>&m- is sometimes called moving FD m to FD n.

This way what the script writes to FD 2 (normally stderr) will be written to stdout because of the second redirection. What the script writes to FD 1 (normally stdout) will be written to stderr because of the first and third redirections. Stdout and stderr got replaced. Done.

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.
output=$(command 2>&1 1>&3)  # Run command.  stderr is captured.
exec 3>&-                    # Close FD #3.
 
# or this alternative, which captures stderr, letting stdout through:
{ output=$(command 2>&1 1>&3-) ;} 3>&1

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

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