The second rule allows you to create chains of commands, each of which uses the output of the
previous command as its input. A typical use of this behavior is a command pipeline, as in the
following rather contrived example:
$ cat /etc/passwd | cut -f 5 -d: | tr [:lower:] [:upper:] | sort | head -5
The first part of the command pipeline, cat /etc/passwd, writes the contents of the /etc/passwd
file to standard output. The second part, cut -f 5 -d:, cuts out the fifth field of its standard
input (the contents of /etc/passwd), using the colon character (:) as the field delimiter (the fifth
field of /etc/passwd is the GECOS or name field).
The third part, tr [:lower:] [:upper:], translates all lowercase characters in the standard
input to uppercase characters. The next element, sort, performs an alphabetic sort on the first letter
of its input before sending the sorted list to standard output. The final component, head -5,
displays only the first five lines of its standard input to standard out. The output of this pipeline
might resemble the following:
ADM
BIN
DAEMON
GAMES
LP
The following command pipeline should prove more useful: it e-mails the current uptime and load
average to the root user:
$ uptime | mailx -s "System Usage" root
The third rule, keeping programs self-contained, is related to the second.
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