13.10. Pipes and Pagers

In Linux, pipes connect the standard output of one command to the standard input of another command.

Consider the ls command that was discussed earlier. There are plenty of options available with ls, but what if the contents of a directory scroll by too quickly for you to view them?

View the contents of the /etc/ directory with the command:

ls -al /etc

How do you get a closer look at the output before it moves off the screen?

One way is to pipe the output to a utility called less, a pager utility that allows you to view information one page (or screen) at a time.

Use the vertical bar (|) to pipe the commands.

ls -al /etc | less

Now you can view the contents of /etc one screen at a time. To move forward a screen, press [Space]; to move back a screen, press [B]; to quit, press [Q]. Alternatively, you can use the arrow keys to navigate with less.

To search the output of a text file using less, press [/] and then type the keyword you want to search for within the file. For example:

/Linux

	Tip
	To read startup messages more closely, at a shell prompt, type dmesg | less. You will be able to read the file one screen at a time. Use the arrow keys to navigate the file; to search for output, press [/] and type the search term.

Pipes can also be used to print only certain lines from a file. Type:

grep coffee sneakers.txt | lpr

This command prints every line in the sneakers.txt file that mentions the word "coffee" (read more about grep in Section 13.11.3 The grep Command).

ls -al /etc | more

/foo