Pipelines

Pipelines, often called pipes, is a way to chain commands and connect output from one command to the input of the next. A pipeline is represented by the pipe character: |. It's particularly handy when a complex or long input is required for a command.

command1 | command2

By default pipelines redirects only the standard output, if you want to include the standard error you need to use the form |& which is a short hand for 2>&1 |.

Example:

Imagine you quickly want to know the number of entries in a directory, you can use a pipe to redirect the output of the ls command to the wc command with option -l.

ls / | wc -l

Then you want to see only the first 10 results

ls / | head

Note: head outputs the first 10 lines by default, use option -n to change this behavior

Grep searches for patterns in each file. Patterns is one or more patterns separated by newline characters, and grep prints each line that matches a pattern. Typically patterns should be quoted when grep is used in a shell command.

ls / | grep  # This will grab any line/file that has a matching pattern in it

Exercise

In this exercise, you will need to print the number of processors based on the information in the cpuinfo file (/proc/cpuinfo)

Hint 1: each processor has a unique number, for instance the first processor will contain the line processor: 0 Hint 2: you can chain together more than two commands in a row