Bash: redirect stderr to file and stdout + stderr to screen

bashio-redirectionlinuxpipestdout

I would like to save the stderr stream of a command into a log file but I also want to display the whole output (stdout + stderr) on the screen. How can I do this?

I only found the solution to display stdout + stderr to the console and redirect both streams to a file as well:

 foo | tee output.file

(https://stackoverflow.com/questions/418896/how-to-redirect-output-to-a-file-and-stdout)

But I only want to redirect stderr to the log file.

Best Answer

With a recent bash, you can use process substitution.

foo 2> >(tee stderr.txt)

This just sends stderr to a program running tee.

More portably

exec 3>&1 
foo 2>&1 >&3 | tee stderr.txt

This makes file descriptor 3 be a copy of the current stdout (i.e. the screen), then sets up the pipe and runs foo 2>&1 >&3. This sends the stderr of foo to the same place as the current stdout, which is the pipe, then sends the stdout to fd 3, the original output. The pipe feeds the original stderr of foo to tee, which saves it in a file and sends it to the screen.

Related Solutions

Bash – Show only stderr on screen but write both stdout and stderr to file

Even without any redirection, or with nothing but >logfile 2>&1, you're not guaranteed to see output in order of generation.

For starters, the stdout from the application will be line-buffered (to tty) or buffered (to a pipeline) but stderr is unbuffered, so relationships between the order of output are broken as far as a reader is concerned. Subsequent stages in any pipeline you could concoct will not get deterministically ordered access to the two streams (they are conceptually things happening in parallel, and you're always subject to the scheduler - if by the time your reader gets a slice the writer has already written to both pipes, you cannot tell which came first).

"[T]he order they happen" is only really known to the application. Ordering of output across stdout/stderr is a well-known - classic, perhaps - problem.

Bash – Use a Variable to Store stderr|stdout Redirection

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.

Best Answer

Related Solutions

Bash – Show only stderr on screen but write both stdout and stderr to file

Bash – Use a Variable to Store stderr|stdout Redirection

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