How do I write standard error to a file while using "tee" with a pipe?

I'm assuming you want to still see standard error and standard output on the terminal. You could go for Josh Kelley's answer, but I find keeping a tail around in the background which outputs your log file very hackish and cludgy. Notice how you need to keep an extra file descriptor and do cleanup afterward by killing it and technically should be doing that in a trap '...' EXIT.

There is a better way to do this, and you've already discovered it: tee.

Only, instead of just using it for your standard output, have a tee for standard output and one for standard error. How will you accomplish this? Process substitution and file redirection:

command > >(tee -a stdout.log) 2> >(tee -a stderr.log >&2)

Let's split it up and explain:

> >(..)

>(...) (process substitution) creates a FIFO and lets tee listen on it. Then, it uses > (file redirection) to redirect the standard output of command to the FIFO that your first tee is listening on.

The same thing for the second:

2> >(tee -a stderr.log >&2)

We use process substitution again to make a tee process that reads from standard input and dumps it into stderr.log. tee outputs its input back on standard output, but since its input is our standard error, we want to redirect tee's standard output to our standard error again. Then we use file redirection to redirect command's standard error to the FIFO's input (tee's standard input).

See Input And Output

Process substitution is one of those really lovely things you get as a bonus of choosing Bash as your shell as opposed to sh (POSIX or Bourne).

In sh, you'd have to do things manually:

out="${TMPDIR:-/tmp}/out.$$" err="${TMPDIR:-/tmp}/err.$$"
mkfifo "$out" "$err"
trap 'rm "$out" "$err"' EXIT
tee -a stdout.log < "$out" &
tee -a stderr.log < "$err" >&2 &
command >"$out" 2>"$err"

To redirect standard error to a file, display standard output to the screen, and also save standard output to a file:

./aaa.sh 2>ccc.out | tee ./bbb.out

To display both standard error and standard output to screen and also save both to a file, you can use Bash's I/O redirection:

#!/bin/bash

# Create a new file descriptor 4, pointed at the file
# which will receive standard error.
exec 4<>ccc.out

# Also print the contents of this file to screen.
tail -f ccc.out &

# Run the command; tee standard output as normal, and send standard error
# to our file descriptor 4.
./aaa.sh 2>&4 | tee bbb.out

# Clean up: Close file descriptor 4 and kill tail -f.
exec 4>&-
kill %1

If using Bash:

# Redirect standard out and standard error separately
% cmd >stdout-redirect 2>stderr-redirect

# Redirect standard error and out together
% cmd >stdout-redirect 2>&1

# Merge standard error with standard out and pipe
% cmd 2>&1 |cmd2

Credit (not answering from the top of my head) goes here: Re: bash : stderr & more (pipe for stderr)

If you're using Z shell (zsh), you can use multiple redirections, so you don't even need tee:

./cmd 1>&1 2>&2 1>out_file 2>err_file

Here you're simply redirecting each stream to itself and the target file.

Full example

% (echo "out"; echo "err">/dev/stderr) 1>&1 2>&2 1>/tmp/out_file 2>/tmp/err_file
out
err
% cat /tmp/out_file
out
% cat /tmp/err_file
err

Note that this requires the MULTIOS option to be set (which is the default).

MULTIOS

Perform implicit tees or cats when multiple redirections are attempted (see Redirection).

Like the accepted answer well explained by lhunath, you can use

command > >(tee -a stdout.log) 2> >(tee -a stderr.log >&2)

Beware than if you use bash you could have some issue.

Let me take the matthew-wilcoxson example.

And for those who "seeing is believing", a quick test:

(echo "Test Out";>&2 echo "Test Err") > >(tee stdout.log) 2> >(tee stderr.log >&2)

Personally, when I try, I have this result:

user@computer:~$ (echo "Test Out";>&2 echo "Test Err") > >(tee stdout.log) 2> >(tee stderr.log >&2)
user@computer:~$ Test Out
Test Err

Both messages do not appear at the same level. Why does Test Out seem to be put like if it is my previous command?

The prompt is on a blank line letting me think the process is not finished, and when I press Enter this fix it. When I check the content of the files, it is ok, and redirection works.

Let’s take another test.

function outerr() {
  echo "out"     # stdout
  echo >&2 "err" # stderr
}

user@computer:~$ outerr
out
err

user@computer:~$ outerr >/dev/null
err

user@computer:~$ outerr 2>/dev/null
out

Trying again the redirection, but with this function:

function test_redirect() {
  fout="stdout.log"
  ferr="stderr.log"
  echo "$ outerr"
  (outerr) > >(tee "$fout") 2> >(tee "$ferr" >&2)
  echo "# $fout content: "
  cat "$fout"
  echo "# $ferr content: "
  cat "$ferr"
}

Personally, I have this result:

user@computer:~$ test_redirect
$ outerr
# stdout.log content:
out
out
err
# stderr.log content:
err
user@computer:~$

No prompt on a blank line, but I don't see normal output. The stdout.log content seem to be wrong, and only stderr.log seem to be ok.

If I relaunch it, the output can be different...

So, why?

Because, like explained here:

Beware that in bash, this command returns as soon as [first command] finishes, even if the tee commands are still executed (ksh and zsh do wait for the subprocesses)

So, if you use Bash, prefer use the better example given in this other answer:

{ { outerr | tee "$fout"; } 2>&1 1>&3 | tee "$ferr"; } 3>&1 1>&2

It will fix the previous issues.

Now, the question is, how to retrieve exit status code?

$? does not work.

I have no found better solution than switch on pipefail with set -o pipefail (set +o pipefail to switch off) and use ${PIPESTATUS[0]} like this:

function outerr() {
  echo "out"
  echo >&2 "err"
  return 11
}

function test_outerr() {
  local - # To preserve set option
  ! [[ -o pipefail ]] && set -o pipefail; # Or use second part directly
  local fout="stdout.log"
  local ferr="stderr.log"
  echo "$ outerr"
  { { outerr | tee "$fout"; } 2>&1 1>&3 | tee "$ferr"; } 3>&1 1>&2
  # First save the status or it will be lost
  local status="${PIPESTATUS[0]}" # Save first, the second is 0, perhaps tee status code.
  echo "==="
  echo "# $fout content :"
  echo "<==="
  cat "$fout"
  echo "===>"
  echo "# $ferr content :"
  echo "<==="
  cat "$ferr"
  echo "===>"
  if (( status > 0 )); then
    echo "Fail $status > 0"
    return "$status" # or whatever
  fi
}

user@computer:~$ test_outerr
$ outerr
err
out
===
# stdout.log content:
<===
out
===>
# stderr.log content:
<===
err
===>
Fail 11 > 0

Thanks lhunath for the answer in POSIX.

Here's a more complex situation I needed in POSIX with the proper fix:

# Start script main() function
# - We redirect standard output to file_out AND terminal
# - We redirect standard error to file_err, file_out AND terminal
# - Terminal and file_out have both standard output and standard error, while file_err only holds standard error

main() {
    # my main function
}

log_path="/my_temp_dir"
pfout_fifo="${log_path:-/tmp}/pfout_fifo.$$"
pferr_fifo="${log_path:-/tmp}/pferr_fifo.$$"

mkfifo "$pfout_fifo" "$pferr_fifo"
trap 'rm "$pfout_fifo" "$pferr_fifo"' EXIT

tee -a "file_out" < "$pfout_fifo" &
    tee -a "file_err" < "$pferr_fifo" >>"$pfout_fifo" &
    main "$@" >"$pfout_fifo" 2>"$pferr_fifo"; exit

Compilation errors which are sent to standard error (STDERR) can be redirected or save to a file by:

Bash:

gcc temp.c &> error.log

C shell (csh):

% gcc temp.c |& tee error.log

See: How can I redirect compilation/build error to a file?