Tuesday, March 7, 2017

Linux Lexicon: Use Watch Command To Run A Command Every X Seconds


watch-command-in-linuxShort Bytes: Have you ever needed to run a command every couple minutes to check on something? Say you need to watch a RAID rebuild or watch a log in real time, but need to search or filter it first. That takes a lot of specialized tools, one for each task really. But using watch command this can be achieved easily.
There is a nifty little command that’s incredibly simple to use, and it’s called watch.
What watch does is it runs the command in a loop, but clears the terminal before running it each subsequent time, and additionally, displays the interval, command, and date/time as the first line. The default interval is two seconds, but this can be manually set using the -n flag with a lower limit of one-tenth of a second.
Here, below, we run the free (a memory usage reporting tool) command every five seconds.
devin@fossbytes$watch -n 5 free -m
Every 5.0s: free -m                                                                                          Sat Sep 24 13:58:24 2016
                               total        used        free      shared  buff/cache   available
    Mem:           257678      39474   170916        4101          47287      208519
    Swap:                7911        1218        6693
As you can see, we were able to pass in the -m (display in megabytes) flag to free without confusing watch. This is because all arguments after the first argument, which is a non-option, are passed to the executed command. This gives you some freedom to pass commands without the need of quotes, though, in the cases where piping and redirection are used, quotation marks will be required otherwise the output of watch will be what’s piped.
There are many options that can be passed to watch, like -t to remove the header information, or -d to highlight the differences between each interval. Below is the full list according to the documentation.
-b, –beep beep if command has a non-zero exit
-c, –color interpret ANSI color and style sequences
-d, –difference highlight changes between updates
-e, –errexit exit if command has a non-zero exit
-g, –chgexit exit when output from command changes
-n, –interval seconds to wait between updates
-p, –precise attempt run command in precise intervals
-t, –no-title turn off header
-x, –exec pass command to exec instead of “sh -c”
-h, –help display help and exit
-v, –version output version information and exit
With these options, it’s easy to see how we can combine watch and a little bit of scripting with other tools (or sysadmin-fu as some like to call it) to create complex monitoring tools that are custom tailored to our specific needs.
Show us how you watch in the comments below.
Also ReadLinux Lexicon — Input And Output With Pipes And Redirection In Linux

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