Bash – Execute command and store everything to variable in bash

To get the output of time into a var use the following:

usr@srv $ mytime="$(time ( ls ) 2>&1 1>/dev/null )"
usr@srv $ echo "$mytime"

real    0m0.006s
user    0m0.001s
sys     0m0.005s

You can also just ask for a single time type, e.g. utime:

usr@srv $ utime="$( TIMEFORMAT='%lU';time ( ls ) 2>&1 1>/dev/null )"
usr@srv $ echo "$utime"
0m0.000s

To get the time you can also use date +%s.%N, so take it before and after execution and calculate the diff:

START=$(date +%s.%N)
command
END=$(date +%s.%N)
DIFF=$(echo "$END - $START" | bc)
# echo $DIFF

You could do (POSIXly):

if { cmd 2>&1 >&3 3>&- | grep '^' >&2; } 3>&1; then
  echo there was some output on stderr
fi

Or to preserve the original exit status if it was non-zero:

fail_if_stderr() (
  rc=$({
    ("$@" 2>&1 >&3 3>&- 4>&-; echo "$?" >&4) |
    grep '^' >&2 3>&- 4>&-
  } 4>&1)
  err=$?
  [ "$rc" -eq 0 ] || exit "$rc"
  [ "$err" -ne 0 ] || exit 125
) 3>&1

Using exit code 125 for the cases where the command returns with a 0 exit status but produced some error output.

To be used as:

fail_if_stderr cmd its args || echo "Failed with $?"