bash-process-tricks

Process Tricks

how to chain commands, run stuff in the background, and generally control what’s happening in your terminal.

chaining

&& — only run the next thing if the first one worked:

make && make install
cd /project && npm start

|| — only run the next thing if the first one FAILED:

cd /mydir || mkdir /mydir
ping -c1 server.com || echo "it's down"

combine them for a ghetto ternary:

test -f config.yml && echo "found it" || echo "nope"

not technically a real ternary (the || fires if the echo fails too, which it won’t, but still). good enough for quick checks.

; just runs stuff in order no matter what:

echo "start"; sleep 2; echo "done"

background jobs

stick & at the end:

./slow-thing.sh &

your terminal is free. jobs to see what’s running, fg to bring it back.

if you accidentally started something without &, hit Ctrl+Z to suspend it (bash-keyboard-shortcuts), then bg to let it keep running in the background.

disown detaches it from your terminal completely — so it survives after you close the tab:

./server.sh &
disown

or just use nohup from the start if you know you want that:

nohup ./server.sh > out.log 2>&1 &

subshells

wrap stuff in () and it runs in its own little world:

(cd /tmp && tar xf thing.tar.gz)
# still in your original directory after this

I use this a lot when I need to cd somewhere temporarily. no cleanup needed.

process substitution

this is a sleeper feature. <() lets you use command output as if it were a file:

diff <(sort file1.txt) <(sort file2.txt)

no temp files. works great for comparing remote stuff too:

diff <(ssh server1 cat /etc/config) <(ssh server2 cat /etc/config)

a few one-liners I keep coming back to

wait for a port:

while ! nc -z localhost 8080; do sleep 1; done; echo "ready"

poor man’s watch:

while true; do clear; date; curl -s localhost/health; sleep 5; done

parallel stuff with xargs:

find . -name "*.png" | xargs -P4 -I{} convert {} {}.webp

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see also: bash-keyboard-shortcuts, bash-quick-loops, bash-history-expansion

greg’s wiki on job control — greg’s wiki is honestly one of the best bash resources out there