# Cheatsheet & Examples: watch

The `watch` command repeatedly executes a program or command and displays its output. This is useful for monitoring the output of a command that changes over time.

## Monitor a Command's Output

Example Usage:
`watch ls -l`

What it does:
This will repeatedly run the `ls -l` command and display its output, updating the display every two seconds (the default interval).

Command-line Arguments Explained:

- `ls -l`: The command to be executed. `ls` lists files, and `-l` provides a long listing format.

## Specifying an Update Interval

Example Usage:
`watch -n 1.5 df -h`

What it does:
This will run the `df -h` command, displaying disk space usage, and refresh the output every 1.5 seconds.

Command-line Arguments Explained:

- `-n 1.5`: Specifies the update interval in seconds.
- `df -h`: The command to be executed. `df` reports disk space usage, and `-h` displays it in human-readable format.

## Highlighting Changes

Example Usage:
`watch -d -n 2 ps aux`

What it does:
This command watches the output of `ps aux`, which lists all running processes. The `-d` option will highlight the differences between updates. The output refreshes every two seconds.

Command-line Arguments Explained:

- `-d`: Highlights the differences between updates.
- `-n 2`: Specifies the update interval to 2 seconds.
- `ps aux`: The command to be executed. `ps` is used to display process information.

## Executing a Command and Ignoring Errors

Example Usage:
`watch --no-title --errexit "ping google.com"`

What it does:
Runs `ping google.com` repeatedly and displays its output without a header or error exit.

Command-line Arguments Explained:

- `--no-title`: Suppresses the header that displays the command and update interval.
- `--errexit`: Exits `watch` if the command being watched exits with a non-zero exit code (an error).
- `ping google.com`: The command to be executed. `ping` sends network packets to a specified host.

## Using a Custom Command to Update the Screen

Example Usage:
`watch -c cat /proc/loadavg`

What it does:
This watches the output of `cat /proc/loadavg` and uses the 'cat' command to render the output to the screen, displaying the system load averages.

Command-line Arguments Explained:

- `-c`: Use command for screen output.
- `cat /proc/loadavg`: The command to be executed. `cat` displays the contents of a file. `/proc/loadavg` provides system load averages.