The | symbol in bash is known as a pipe. It is simple but powerful. It is used to pipe the output of one process into the input of another. In this way it reminds me of the functional programming style of writing simple, single purpose functions, and chaining them together to perform complex tasks.

On the command line it becomes easy to write simple programs using the pipe.

For example, let’s get the disk usage for all the files in the /Downloads directory, then sort them from high to low. We will pipe the output of du aka disk usage into the sort tool.

du -h ./Downloads | sort -hr

-h for human-readable output (aka megabytes instead of bytes) -r for reverse sort (high to low)

if the output is too much to see on one screen, send it into less so you can scroll thru it.

du -h ./Downloads | sort -hr | less

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Another useful chain is piping stuff into grep.

See if a file called “my_file” exists in the current directory:

ls | grep my_file

find . -name my_file also does this job, but I prefer grep as I don’t have to use wildcards for partial matching.

Anything that produces a text output can be fed into grep in this manner.

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When I’m writing these blog posts, I run the Jekyll server in the background to check formatting before I commit and push. This means I have to locate the process and kill it when I’m done. Finding and killing a process is a great example of a chain of commands that can be piped together to form a simple program.

Here’s the chain in plain English:

  • print list of all running processes
  • find the ones that contain “jekyll”
  • filter out the grep process from the list, leaving only the actual jekyll process
  • get the pid for the jekyll process which is located in the 2nd column of the whitespace-separated output
  • kill that process

and in bash:

# WARNING! don't run kill commands copied from the internet on your system!

ps aux | grep jekyll | grep -v grep | awk  '{print $2}' | xargs kill -9

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Piping is a great way to learn the basics of programming pipelines and connecting inputs to outputs. A simple but powerful idea that can be taken to great heights.