bash script print stderr on failure

Question

I am trying to make the bash script silent if everything goes fine but print out all the stderr and debug info if it crashes for some reason. Below is what I have so far.

#!/usr/bin/bash

set -e

rm -f /tmp/err
trap "sleep 1 && cat /tmp/err" ERR

l() {
    ts >> /tmp/err
}

echo "About to download stuff:" > >(l)
# curl blah blah 2> >(l)

# something goes wrong in the script
invalid_cmd

It works fine only if I have the 'sleep 1' which I don't like.

Without sleep:

❯ ./demo2.sh    
./demo2.sh: line 18: invalid_cmd: command not found

With sleep:

❯ ./demo2.sh
./demo2.sh: line 18: invalid_cmd: command not found
Feb 25 15:20:44 About to download stuff:

I think that is because the process substitution runs in the background and may not complete. I also don't want to blindly wait for all background tasks. Is there a better way to fix this?

if you don't want to blindly wait for all background tasks, wait, then wait for the ones you care about: Capture the PID of a backgrounded task from $! with e. g. mypid="$!", and then later wait "${mypid}". — DopeGhoti
– DopeGhoti, Commented Feb 25, 2022 at 21:58
Good possibility. I usually just stuff the results of every command in a general environment variable or temporary file and display/log that when something fails. — JdeHaan
– JdeHaan, Commented Feb 26, 2022 at 9:50

balki · Accepted Answer · 2022-02-27 16:48:42Z

After some more hacking, found below solution.

#!/usr/bin/bash

set -e

make_temp_fds() {
    local tfile=$(mktemp)
    exec {fdw}>$tfile {fdr1}<$tfile {fdr2}<$tfile
    rm $tfile
}
    
setup_err_fd() {
    make_temp_fds
    local raw_fdw=$fdw raw_fdr=$fdr1

    make_temp_fds
    local ts_fdw=$fdw ts_fdr=$fdr1

    ts <&$raw_fdr >&$ts_fdw &
    tspid=$!
    trap "wait $tspid && cat <&$ts_fdr" ERR

    err_fd=$raw_fdw
}

# Calling this function, will give you $err_fd
# Anything written to $err_fd will be saved in a temporary file with timestamps 
# and printed out if the script fails with an error. 
# Will be lost if script exits successfully
setup_err_fd

echo "all setup"

echo "debug line 1" >&$err_fd

echo "debug line 2" >&$err_fd

not_ok
#echo "ok"

echo "bye"

Unfortunately it involves two temporary files. I tried to make one of them a pipe but couldn't get it working. Feel free to improve

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