BASH Cures Cancer

I realized today, that it may not be obvious that command substitution can be done in parallel with compound commands. Meaning that variable assignment and tests can be done inline. For example, the exit status of the command inside an assignment operation is the exit value of the assignment operation:

$ val=$( false ); echo $?
1
$ val=$( true ); echo $?
0

I used this today when I wrote a long running test which slept 60 seconds between loops. Midway through the test I decided I wanted it to sleep only 15 seconds. Here was my command to resolve this situation without stopping my test:

$ while :; do p=$(pgrep sleep) && sleep 15 && kill $p; sleep 1;done

If pgrep finds and any “sleep” process, the command sleeps 15 seconds and then kills the previously described process. After which it sleeps one second and looks for more “sleep” processes to kill. I am assigning the output of pgrep to the variable p and using pgrep’s exit status to decided whether any processes need to be killed.

The alternative would have been something like:

$ while :; do p=$(pgrep sleep); [[ ! -z "$p" ]] && sleep 15 && kill $p; sleep 1;done