Bash Patterns I Use Weekly

1. Find and replace a pattern in a codebase with capture groups

git grep -l pattern | xargs gsed -ri 's|pat(tern)|\1s are birds|g'

• git grep -l: make sure we're only looking for files in our codebase (ag -l is another good option)
• xargs: allow running this with gsed -i
• gsed -i: edit files (default mac sed is bad, so gnu-sed is essential)
• gsed -r: use regular expression for the pattern to allow capture groups
• s|: the first character after the s is used as the delimiter. / is pretty consistently annoying because it's used in file paths & urls.
• gsed '...': the single quotes are important to avoid escapes
• 's|pat(tern)|\1s are birds|g': being able to easily use capture groups in a find and replace is amazing
• |g: replace this multiple times in a line

2. Track down a commit when a command started failing

while command; do git checkout HEAD^; done;

• while command: As long as command exits successfully (with a 0), this loop will keep going. (this is why while true; works! The same thing works with if: if git grep yay; then echo "found yay"; fi)
• git checkout HEAD^: go back in time.
• git bisect is the "real" way to do this, but it's not something I've ever needed, and while command and while ! command are both generally useful to know
• A similar pattern to poll until something succeeds could be while ! curl localhost:8080; do sleep 1; done;.

3. Parallelize running commands by grabbing PIDs.

pids="";
do_thing_1 &
pids="$pids $!"
do_thing_2 &
pids="$pids $!"
EXIT_CODE=0
for p in $pids; do
  if ! wait $p; then EXIT_CODE=1; fi
done

exit $EXIT_CODE;

4. Use $SECONDS to track how long things take

echo "Your command completed after $SECONDS seconds";

• Try running this in your shell: SECONDS=0; sleep 5; echo $SECONDS;
• I often prefer this over time when setting up logs for a script

5. Use for to iterate over simple lists:

for route in foo bar baz; do
  curl localhost:8080/$route
done

The default separator in bash is a space, and you can take advantage of this to make simple for loops. (This default is quite frustrating to work around in general: spaces in file names are the devil.)