Redirect stderr and stdout in Bash

Question

I want to redirect both stdout and stderr of a process to a single file. How do I do that in Bash?

Answer 1

# Close STDOUT file descriptor
exec 1<&-
# Close STDERR FD
exec 2<&-

# Open STDOUT as $LOG_FILE file for read and write.
exec 1<>$LOG_FILE

# Redirect STDERR to STDOUT
exec 2>&1

echo "This line will appear in $LOG_FILE, not 'on screen'"

Now, simple echo will write to $LOG_FILE. Useful for daemonizing.

To the author of the original post,

It depends what you need to achieve. If you just need to redirect in/out of a command you call from your script, the answers are already given. Mine is about redirecting within current script which affects all commands/built-ins(includes forks) after the mentioned code snippet.

---

Another cool solution is about redirecting to both std-err/out AND to logger or log file at once which involves splitting "a stream" into two. This functionality is provided by 'tee' command which can write/append to several file descriptors(files, sockets, pipes, etc) at once: tee FILE1 FILE2 ... >(cmd1) >(cmd2) ...

exec 3>&1 4>&2 1> >(tee >(logger -i -t 'my_script_tag') >&3) 2> >(tee >(logger -i -t 'my_script_tag') >&4)
trap 'cleanup' INT QUIT TERM EXIT


get_pids_of_ppid() {
    local ppid="$1"

    RETVAL=''
    local pids=`ps x -o pid,ppid | awk "\\$2 == \\"$ppid\\" { print \\$1 }"`
    RETVAL="$pids"
}


# Needed to kill processes running in background
cleanup() {
    local current_pid element
    local pids=( "$$" )

    running_pids=("${pids[@]}")

    while :; do
        current_pid="${running_pids[0]}"
        [ -z "$current_pid" ] && break

        running_pids=("${running_pids[@]:1}")
        get_pids_of_ppid $current_pid
        local new_pids="$RETVAL"
        [ -z "$new_pids" ] && continue

        for element in $new_pids; do
            running_pids+=("$element")
            pids=("$element" "${pids[@]}")
        done
    done

    kill ${pids[@]} 2>/dev/null
}

So, from the beginning. Let's assume we have terminal connected to /dev/stdout(FD #1) and /dev/stderr(FD #2). In practice, it could be a pipe, socket or whatever.

• Create FDs #3 and #4 and point to the same "location" as #1 and #2 respectively. Changing FD #1 doesn't affect FD #3 from now on. Now, FDs #3 and #4 point to STDOUT and STDERR respectively. These will be used as real terminal STDOUT and STDERR.
• 1> >(...) redirects STDOUT to command in parens
• parens(sub-shell) executes 'tee' reading from exec's STDOUT(pipe) and redirects to 'logger' command via another pipe to sub-shell in parens. At the same time it copies the same input to FD #3(terminal)
• the second part, very similar, is about doing the same trick for STDERR and FDs #2 and #4.

The result of running a script having the above line and additionally this one:

echo "Will end up in STDOUT(terminal) and /var/log/messages"

...is as follows:

$ ./my_script
Will end up in STDOUT(terminal) and /var/log/messages

$ tail -n1 /var/log/messages
Sep 23 15:54:03 wks056 my_script_tag[11644]: Will end up in STDOUT(terminal) and /var/log/messages

If you want to see clearer picture, add these 2 lines to the script:

ls -l /proc/self/fd/
ps xf