Justin Joyce

Practical tips and tutorials about software development.

The Bash trap command

Bash’s trap command is used to catch and react to signals sent to your shell. It’s similar to an event listener in the browser, or a pubsub topic subscriber.

The typical use case is to run some kind of cleanup command when a process terminates, like this:

trap cleanup_job TERM SIGINT

The command above will run cleanup_job when it receives a TERM or SIGINT (ctrl-c).

I’ve only needed the trap command a few times, and I’ll go through one example here.

Using trap to kill a background task

At my day job, we have a two-part script to connect to remote databases. The script starts a proxy service and then uses that proxy to query a remote database directly.

We need this workflow most often in emergencies, so it’s setup to be run as a single command for ease-of-use, like so:

# connect_to_db.sh

echo "Starting Sql Proxy"

# Use bash '&' to run this in the background
cloud_sql_proxy -instance=lots-of-info-here &

# Store the process id of the proxy command
PROXY_PID=$!

# Kill the proxy process when we exit the postgres shell
# This won't actually run until it hears a TERM or SIGINT
trap "echo '===Terminating sql proxy $PROXY_PID===' && kill $PROXY_PID" TERM SIGINT

echo "connecting to DB"

# Not the prettiest, but has never failed us
sleep 3

# Connect to the database through the proxy
PGPASSWORD=secret psql -h 127.0.0.1 -p 1337 -U username -d dbname

After running connect_to_db.sh, I’m dropped into a psql shell in the remote database. As long as I’m in there the proxy keeps running, but as soon as I end my connection the trap command will receive its TERM or SIGINT signal-depending on how I exited-and it’ll react by echoing a line in the terminal and running kill $PROXY_PID.

The script above also makes use of a few other bash operators and parameters like &, &&, and $!. I’ve added a few links below with more information on each of those.

Helpful Links