Capture both stdout and stderr in Bash
This question already has an answer here:
There is no way to capture both without temp file.
You can capture stderr to variable and pass stdout to user screen (sample from here):
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
But there is no way to capture both 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.
There's a really ugly way to capture stderr and stdout in two separate variables without temporary files (if you like plumbing), using process substitution, source , and declare appropriately. I'll call your command banana . You can mimic such a command with a function:
banana() {
echo "banana to stdout"
echo >&2 "banana to stderr"
}
I'll assume you want standard output of banana in variable bout and standard error of banana in variable berr . Here's the magic that'll achieve that (Bash≥4 only):
. <({ berr=$({ bout=$(banana); } 2>&1; declare -p bout >&2); declare -p berr; } 2>&1)
So, what's happening here?
Let's start from the innermost term:
bout=$(banana)
This is just the standard way to assign to bout the standard output of banana , the standard error being displayed on your terminal.
Then:
{ bout=$(banana); } 2>&1
will still assign to bout the stdout of banana , but the stderr of banana is displayed on terminal via stdout (thanks to the redirection 2>&1 .
Then:
{ bout=$(banana); } 2>&1; declare -p bout >&2
will do as above, but will also display on the terminal (via stderr) the content of bout with the declare builtin: this will be reused soon.
Then:
berr=$({ bout=$(banana); } 2>&1; declare -p bout >&2); declare -p berr
will assign to berr the stderr of banana and display the content of berr with declare .
At this point, you'll have on your terminal screen:
declare -- bout="banana to stdout"
declare -- berr="banana to stderr"
with the line
declare -- bout="banana to stdout"
being displayed via stderr.
A final redirection:
{ berr=$({ bout=$(banana); } 2>&1; declare -p bout >&2); declare -p berr; } 2>&1
will have the previous displayed via stdout.
Finally, we use a process substitution to source the content of these lines.
You mentioned the return code of the command too. Change banana to:
banana() {
echo "banana to stdout"
echo >&2 "banana to stderr"
return 42
}
We'll also have the return code of banana in the variable bret like so:
. <({ berr=$({ bout=$(banana); bret=$?; } 2>&1; declare -p bout bret >&2); declare -p berr; } 2>&1)
You can do without sourcing and a process substitution by using eval too (and it works with Bash<4 too):
eval "$({ berr=$({ bout=$(banana); bret=$?; } 2>&1; declare -p bout bret >&2); declare -p berr; } 2>&1)"
And all this is safe, because the only stuff we're source ing or eval ing are obtained from declare -p and will always be properly escaped.
Of course, if you want the output in an array (eg, with mapfile , if you're using Bash≥4—otherwise replace mapfile with a while – read loop), the adaptation is straightforward.
For example:
banana() {
printf 'banana to stdout %dn' {1..10}
echo >&2 'banana to stderr'
return 42
}
. <({ berr=$({ mapfile -t bout < <(banana); } 2>&1; declare -p bout >&2); declare -p berr; } 2>&1)
and with return code:
. <({ berr=$({ mapfile -t bout< <(banana; bret=$?; declare -p bret >&3); } 3>&2 2>&1; declare -p bout >&2); declare -p berr; } 2>&1)
You can do:
OUT=$(myscript.sh 2> errFile)
ERR=$(<errFile)
Now $OUT will have standard output of your script and $ERR has error output of your script.
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