How do I write stderr to a file while using “tee” with a pipe?

I have the below command line argument which will print the output of aaa.sh to the screen while also writing stdout to bbb.out; however I would also like to write stderr to a file ccc.out. Any suggestions on how to modify the below piece? Thanks!

./aaa.sh | tee ./bbb.out

Update: stdout and stderr should still both be printed to the screen, regardless.

 

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

command >>(tee stdout.log)2>>(tee 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 STDOUT of command to the FIFO that your first tee is listening on.

Same thing for the second:

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

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

See http://mywiki.wooledge.org/BashGuide/InputAndOutput

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 stdout.log <"$out"&
tee stderr.log <"$err">&2&
command >"$out"2>"$err"

 I tried this: $ echo "HANG" > >(tee stdout.log) 2> >(tee stderr.log >&2) which works

 

 

 

why not simply:

./aaa.sh 2>&1| tee -a log

This simply redirects stderr to stdout, so tee echoes both to log and to screen. Maybe I'm missing something, because some of the other solutions seem really complicated.

That works fine if you want both stdout (channel 1) and stderr (channel 2) logged to the same file (a single file containing the mixture of both stdout and sterr). The other, more complicated solution allows you to separate stdout and stderr into 2 different files (stdout.log and stderr.log, respectively). Sometimes that is important, sometimes it's not.

The problem with this method is that you lose the exit/status code from the aaa.sh process, which can be important (e.g. when using in a makefile)??

 

 

To redirect stderr to a file, display stdout to screen, and also save stdout to a file:

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

EDIT: To display both stderr and stdout 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 stderr.
exec 4<>ccc.out

# Also print the contents of this file to screen.
tail -f ccc.out &# Run the command; tee stdout as normal, and send stderr# 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