watch: the poor man's monitoring, in one command

Re-run it until something changes

Everyone has written this loop, in some form:

bash

while true; do clear; df -h /var; sleep 2; done

Re-run a command, clear the screen, wait, repeat. It works, and watch does the same in eleven characters, plus a handful of things the loop cannot do at all.

bash

watch df -h /var

What you get for free

The default interval is two seconds, and -n changes it:

bash

watch -n 1 df -h /var          # every second
watch -n 0.5 'ls -l | wc -l'   # twice a second

Note the quotes on the second one. watch passes the command to sh -c, so an unquoted pipe would be interpreted by your shell and only ls -l would be watched. Quote anything with a pipe, a redirection or a semicolon. If you would rather skip the shell entirely, -x execs the command directly.

The header shows the interval, the command and the host, which sounds cosmetic until you have four of these open and no idea which is which. -t removes it when you want the screen for the output.

The flag that changes how you use the tool is -d, which highlights what changed since the previous run:

bash

watch -d -n 1 'ss -tn state established | wc -l'

Your eye stops scanning the whole screen and goes to the parts that moved. With -d=permanent, anything that has ever changed stays highlighted, so you can look away and still see what happened.

The part nobody knows: it can stop on its own

watch is usually described as an infinite loop. A shame, because three flags turn it into a condition to wait for.

-g exits as soon as the output changes:

bash

watch -g -n 5 'systemctl is-active myservice'

That blocks until the service state moves, then returns. It’s a wait-for-it primitive without a polling script.

-e exits when the command itself fails:

bash

$ timeout 5 watch -n 1 -e false

$ echo $?
1

It stopped on the first failure instead of running for the full five seconds. Handy to babysit something that should keep succeeding, and to know the moment it does not.

And -q <cycles> is the mirror image: exit when the output has not changed for that many cycles. Waiting for a queue to stabilise, or a file to stop growing, is a one-liner:

bash

watch -q 3 -n 2 'ls -l big.iso | awk "{print \$5}"'

Three identical readings and it returns. I keep meeting scripts that reimplement this with a counter and a comparison.

It really does need a terminal

watch draws with ncurses, so it wants a real terminal and says so when it does not have one:

bash

$ watch -n 1 date
ncurses: cannot initialize terminal type ($TERM="unknown"); exiting

That is not a bug to work around, it’s a design decision: the tool exists to redraw a screen. In a script or a CI job, the loop you were trying to replace is the right answer, or -g/-q if you only need the exit condition and can give it a TERM.

Not on macOS

bash

$ command -v watch
$

watch comes from procps-ng, the Linux process tools, version 4.0.4 on my Ubuntu 25.10 box. macOS ships none of that family:

bash

brew install watch

Without Homebrew, the loop from the first paragraph is the fallback, and clear plus sleep gets you most of the way. What you lose is -d, the flag that actually makes watch worth installing.