Shell – A command that generates both standard error and output

Another solution could be the following:

#!/bin/bash

verbose=0

exec 3>&1
exec 4>&2

if ((verbose)); then
  echo "verbose=1"
else
  echo "verbose=0"
  exec 1>/dev/null
  exec 2>/dev/null
fi

echo "this should be seen if verbose"
echo "this should always be seen" 1>&3 2>&4

Then add 1>&3 2>&4 only to commands of which you want to see the output.

In bash it's a bit tricky:

command 2> >(tee err) 1> >(tee out) | tee >all

Here we need process substitution >(...) and tee to work around the problem. With process substitution the tee process is attachted to the corresponding channel. tee then writes the lines into the file and prints then to the STDOUT. So after writing to the files, both STDERR and STDOUT will be printed to STDOUT. In bash, we cannot use multiple redirections of the same chanel in one command. That's why we need to pipe that output to tee again, which then just prints it.

Within zsh (notice the option MULTIOS must be set, which is detault in zsh):

command >out 2>err &>all