I want to write a shell script that runs a command, writing its stderr to my terminal as it arrives. However, I also want to save stderr to a variable, so I can inspect it later.
How can I achieve this? Should I use tee, or a subshell, or something else?
I've tried this:
# Create FD 3 that can be used so stdout still comes through
exec 3>&1
# Run the command, piping stdout to normal stdout, but saving stderr.
{ ERROR=$( $@ 2>&1 1>&3) ; }
echo "copy of stderr: $ERROR"
However, this doesn't write stderr to the console, it only saves it.
I've also tried:
{ $@; } 2> >(tee stderr.txt >&2 )
echo "stderr was:"
cat stderr.txt
However, I don't want the temporary file.
Credit goes to @Etan Reisner for the fundamentals of the approach; however, it's better to use tee with /dev/stderr rather than /dev/tty in order to preserve normal behavior (if you send to /dev/tty, the outside world doesn't see it as stderr output, and can neither capture nor suppress it):
Here's the full idiom:
exec 3>&1 # Save original stdout in temp. fd #3.
# Redirect stderr to *captured* stdout, send stdout to *saved* stdout, also send
# captured stdout (and thus stderr) to original stderr.
errOutput=$("$@" 2>&1 1>&3 | tee /dev/stderr)
exec 3>&- # Close temp. fd.
echo "copy of stderr: $errOutput"