4.11.1.2. Piping

A pipe is a mechanism used to connect the standard output of one program to the standard input of another program. To create a pipe, insert the vertical-bar ( | ) symbol between the two commands:

$ mount

/dev/mapper/main-root on / type ext3 (rw)

proc on /proc type proc (rw)

sysfs on /sys type sysfs (rw)

devpts on /dev/pts type devpts (rw,gid=5,mode=620)

/dev/hdc2 on /boot type ext3 (rw)

tmpfs on /dev/shm type tmpfs (rw)

/dev/mapper/main-home on /home type ext3 (rw)

none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)

/dev/sdb on /media/disk type vfat (rw,noexec,nosuid,nodev,shortname=winnt,uid=503)

$ mount | grep /dev/mapper

/dev/mapper/main-root on / type ext3 (rw)

/dev/mapper/main-home on /home type ext3 (rw)

In this example, the output of the mount command is used as the input to the grep command, which outputs only lines that match the specified pattern. A group of commands connected together with pipe symbols is known as a pipeline . You can extend a pipeline by connecting additional commands:

$ mount | grep /dev/mapper | sort

/dev/mapper/main-home on /home type ext3 (rw)

/dev/mapper/main-root on / type ext3 (rw)

The input to a pipeline and the output from a pipeline may be redirected:

$ cut -d: -f1 output

$ cat output

adm

apache

avahi

beaglidx

bin

chip

chris

daemon

dbus

distcache

However, it's essential that the input redirect take place at the start of the pipeline (at the command on the left) and that the output redirection take place at the end (at the command on the right). Consider this wrong example:

$ cut -d: -f1 output |head

In this case, it's unclear whether the standard output of sort should be placed in the file output or used as the standard input to the head command. The result is undefined (which means don't do this! ).