Hide the output of a shell command only on success?

Question

Hiding the output of a shell command usually involves redirecting stderr and stdout. Is there any builtin facility or command which by default hides the output but on error dumps all the accumulated output? I would like to run this as a wrapper for remote ssh commands. Now I have them using redirection but I don't get a clue as to what made them fail, and they are just too verbose.

EDIT: In the end I created the following template based on the answer by @Belmin which I tweaked a little bit to accumulate all the previous commands from the script, use the current process identifier, automatically remove the log, and add a failure red error message when something goes wrong. In this template the initial silent wrappers will succeed, then fail the third command because the directory already exists:

#!/bin/sh

set -e

SILENT_LOG=/tmp/silent_log_$$.txt
trap "/bin/rm -f $SILENT_LOG" EXIT

function report_and_exit {
    cat "${SILENT_LOG}";
    echo "\033[91mError running command.\033[39m"
    exit 1;
}

function silent {
    $* 2>>"${SILENT_LOG}" >> "${SILENT_LOG}" || report_and_exit;
}

silent mkdir -v pepe
silent mkdir -v pepe2
silent mkdir -v pepe
silent mkdir -v pepe2

Answer 1

It should be easy enough to write a script for this purpose.

Something like this completely untested script.

OUTPUT=`tempfile`
program_we_want_to_capture &2>1 > $OUTPUT
[ $? -ne 0 ]; then
    cat $OUTPUT
    exit 1
fi
rm $OUTPUT

On the other hand for commands I run as part of a script I usually want something better than simply print all the output. I often limit what I see to the unknown. Here is a script I adapted from something I read over a decade ago.

#!/bin/bash

the_command 2>&1 | awk '
BEGIN \
{
  # Initialize our error-detection flag.
  ErrorDetected = 0
}
# Following are regex that will simply skip all lines
# which are good and we never want to see
/ Added UserList source/ || \
/ Added User/ || \
/ init domainlist / || \
/ init iplist / || \
/ init urllist / || \
/ loading dbfile / || \
/^$/ {next} # Uninteresting message.  Skip it.

# Following are lines that we good and we always want to see
/ INFO: ready for requests / \
{
  print "  " $0 # Expected message we want to see.
  next
}

# any remaining lines are unexpected, and probably error messages.  These will be printed out and highlighted.
{
  print "->" $0 # Unexpected message.  Print it
  ErrorDetected=1
}

END \
{
  if (ErrorDetected == 1) {
    print "Unexpected messages (\"->\") detected in execution."
    exit 2
  }
}
'
exit $?

Answer 2

Don't reinvent the wheel on this one. It's a common problem, notably with cron jobs, which is why cronic/chronic were created. You can likely install one from your package manager, and the usage is as simple as this:

cronic noisy_command --param paramval

It will suppress all output unless the noisy command produces non-trace error output or has a non-zero exit status.

Answer 3

I don't think there is a clean way of doing this, the only thing I can think of is

• Capture the output of the command.
• Check the return value of the command and if it failed
  • display the captured output.

Implementing this might though be a interesting project but perhaps beyond Q&A.

Answer 4

I'd setup a bash function like this:

function suppress
{
   /bin/rm --force /tmp/suppress.out 2> /dev/null; \
   ${1+"$@"} > /tmp/suppress.out 2>&1 || \
   cat /tmp/suppress.out; \
   bin/rm /tmp/suppress.out;
}

Then, you could just run the command:

suppress foo -a bar

Answer 5

Try so:

out=`command args...` || echo $out

Answer 6

going short with something like tehcommand &>/tmp/$$ || cat /tmp/$$

depends how much usability/typing you want/need. (e.g. using it as a pipe or passing the command by argument)

@zoredache short script is basically a proto-wrapper for this, which would give more robustness, handle concurrency, etc