par

par is a bash script to run multiple commands in parallel and to output the results color-coded and without lines interleaving.

• Executes programs in parallel
• Human-readable output:
  • Color-coded lines by process. Up to 6 colors: red, green, yellow, purple, pink, or cyan
  • Prefixed lines with process name. Up to 72 characters (or truncated)
  • Prevents interleaving process outputs. Side effect: truncates lines at about 4000 characters
  • Combines stdout and stderr file descriptors for each process
• Kills all processes if any one exits with a nonzero exit code. Includes children one level deep. Otherwise waits for all processes to exit zero

The combined output is inspired by Docker compose's color-coding. The interleaving prevention is adapted from http://catern.org/pipes.html's approach.

Tests

Unit tests must be executed from inside the tests directory since they expect it to be the current working directory. For example run make test or:

cd tests && ./test_crash.sh

The tests are demonstrations and have no assertions. Tests pass when they terminate and fail if they don't. All tests should print "Done" on the last line. Additionally, running ps -ef | grep par should yield no instances of the tests afterwards.

Examples

There are several ways to run commands in parallel. Three approaches are shown using par, xargs, and email. Here's an example with par:

par 'ping google.com' 'ping apple.com'

Another approach, is to use xargs (good for very quick tasks in parallel but interleaves output):

echo '"ping google.com | head -n5" "ping apple.com | head -n5"' | xargs -n1 -P4 -I {} sh -c "echo '{}'; {}"

A third alternative, is to start each process manually and email the results when finished (good for very long tasks i.e. days and doesn't interleave output):

function fork {
    nohup sh -c "$1 | mail -s \"`hostname`,`date`,$1\" email@example.com" &
}
fork 'ping google.com | head -n5'
fork 'ping apple.com | head -n5'