watch command in Linux: periodically repeat a command, examples

Q: A pipeline/grep "doesn't work" inside watch

Wrap the whole command in quotes: watch 'cmd | grep x'. Without quotes, the shell parses |/* before watch even sees them.

Q: Colors disappeared

By default watch strips ANSI color. Turn it on: watch -c 'cmd' (if the command colors its own output).

Q: Need to follow a log line by line

For logs, tail -f (see files-basics) or journalctl -f (see journalctl) are better; watch is for a "snapshot of state", not a stream.

Q: Want to stop on a change / on readiness

watch -g cmd exits once the output changes (handy as "wait until ready"); watch -e cmd stops if the command errors out.

Overview

watch repeats a command every N seconds and shows the fresh result on the same screen — handy for watching a process (a RAID rebuild, a queue, a file's size).

bash

watch command                     # every 2 seconds (the default)
watch -n1 command                 # every second
watch -d command                  # highlight what changed

Options

6

/

Always quote a command that has a pipeline or a wildcard (|, *) — otherwise the shell parses them before watch even runs, instead of the repeated command doing so. By default watch strips ANSI color from the output; -c turns it back on for commands that color their own text.

Option	What it does	Example
`-n` SECONDS	Interval between runs in seconds; fractional values like 0.5 are allowed.	`watch -n1 'ss -tlnp \| grep :80'`
`-d`	Highlight what changed compared to the previous output.	`watch -d free -h`
`-g`	Exit as soon as the command's output changes.	`watch -g 'systemctl is-active nginx'`
`-e`	Stop if the command exits with a non-zero status.	`watch -e 'curl -sf http://localhost/healthz'`
`-c`	Interpret ANSI color codes in the command's output.	`watch -c 'docker ps'`
`-t`	Hide the header showing the command name and time.	`watch -t 'df -h'`

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Recipes

1

Examples

bash

watch -n1 'ss -tlnp | grep :80'   # watch listening ports (see ss)
watch -d free -h                  # memory changes (highlighted)
watch -d 'ps aux --sort=-%cpu | head'   # top by CPU, live
watch cat /proc/mdstat            # RAID rebuild progress (see mdadm)
watch -n5 'ls -lh /var/log/app.log'   # watch a log grow
watch -g 'systemctl is-active nginx'  # wait until the status changes
watch -c 'docker ps'              # with color, container states

Quote a command that has a pipeline/wildcard (watch 'ss | grep x'), otherwise | and * apply to watch itself, not the command.

Troubleshooting

4

A pipeline/grep "doesn't work" inside watch

Wrap the whole command in quotes: watch 'cmd | grep x'. Without quotes, the shell parses |/* before watch even sees them.

Colors disappeared

By default watch strips ANSI color. Turn it on: watch -c 'cmd' (if the command colors its own output).

Need to follow a log line by line

For logs, tail -f (see files-basics) or journalctl -f (see journalctl) are better; watch is for a "snapshot of state", not a stream.

Want to stop on a change / on readiness

watch -g cmd exits once the output changes (handy as "wait until ready"); watch -e cmd stops if the command errors out.

watch — periodically repeat a command

Cheat sheet