Pump command output as function argument

Question

I have this extremely simple function in my script:

# Used for debug tracing.
log()
{
    :
    echo "log: $1"
}

The idea is to be able to customize/turn off logging at a single place. Very crude.

Now I want my script to produce absolutely no output when in release configuration. The only solution I have thought of, but extremely unDRY:

TMPFILE='/tmp/tempfilewithpossiblyuniquename'
cmd 1>"$TMPFILE" 2>"$TMPFILE"
cat "$TMPFILE" | xargs log
rm "$TMPFILE"

for every single command. How to improve on this?

EDIT: I want to collect all output to stdout and stderr and channel it through log(). Then log() can choose to disregard everything, to log to a file, to print etc.

ilkkachu · Answer 1 · 2018-05-29T16:43:44.947

Absolutely no output is simple, just redirect the script's stdout and stderr to /dev/null:

exec >/dev/null 2>&1

That will affect the shell itself, and any commands it executes afterwards. Run that conditionally to choose where the output is redirected.

if [ "$output_to_file" = 1 ]; then
    exec > "$outputfilename" 2>&1
elif [ "$output_suppress" = 1 ]; then
    exec > /dev/null 2>&1
fi

Note, that suppressing all output isn't probably a good idea. The user is very likely to want some notification for errors.

If you insist on passing output through the function (and are running Bash/ksh/Zsh), you could use process substitution:

#!/bin/bash
mangle_output() {
    # do something smarter here
    while read -r line; do
        echo "output: $line";
    done;
}
# redirect stdout and stderr to the function
exec > >(mangle_output) 2>&1
echo something that produces output

Though note that processing the output with a shell loop isn't a very good idea, in the least it's slow. See: Why is using a shell loop to process text considered bad practice?. If all you want is redirection a file, or to /dev/null, just use exec to set up redirections.

Sorry for the ambiguous question. What I wanted to say, it that all output should be routed through log(). — Vorac, May 29 '18 at 16:33

DopeGhoti · Answer 2 · 2018-05-29T16:07:13.220

Firstly, that function will only log the first "word" of anything sent to it, since you use $1 rather than "$*".

Secondly, there are (as is oft the case with POSIX) myriad ways to do this sort of thing. I would probably go with something like:

log() {
    cat - >> "$logfile"
}

do_stuff | log

But you could also:

(
    do_stuff
    do_more_stuff
) >> "$logfile"

As for completely suppressing all output -- that is something that is usually best left for the invoking environment (e. g. ./thing 1> /dev/null 2> &1) rather than locking it down "in code" as it were. That said:

squashout="true"  # comment this out to stop killing output
if ! [[ "true" = "${squashout-false}" ]]; then 
  # Redirect stdout and stderr to the null device.  
  exec 1> /dev/null
  exec 2> /dev/null
fi

Pump command output as function argument

2 Answers2