$ 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 </etc/passwd|sort|head > 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 </etc/passwd|sort > 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! ).
4.11.2. How Does It Work?
Redirection is set up by the bash shell before the command is executed. If there is a redirection error (such as an invalid filename or a file permission problem), it will be reported by the shell and the command will not be executed:
$ cal > foo/bar/baz
bash: foo/bar/baz: No such file or directory
Note that the error message starts with bash, indicating that it was produced by the shell and not by the cal command.
A command is not aware of file redirection unless it has specifically been programed to check the standard file descriptors or perform special operations on them (such as changing terminal characteristics). Redirected file descriptors are inherited by applications that were started by commands; in this example, the nice command starts the cal command, and cal inherits the redirection set up for nice :
$ nice "cal" > test.txt
$ cat test.txt
July 2006
Su Mo Tu We Th Fr Sa
1
2 3 4 5 6 7 8
9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29
30 31
4.11.3. What About...
4.11.3.1. ...redirecting standard output and standard error to the same destination?
You can use the characters 2>&1 to redirect standard error to the same destination as standard output:
$ cal 17 2009 > /tmp/calresult 2>&1
Notice that the order of the redirections matters. The preceding command will redirect all output to /tmp/calresult , but this command will not redirect standard error:
$ cal 17 2009 2>&1 > /tmp/calresult
The 2>&1 redirection is evaluated first, so standard error is directed to the same destination as standard output (which, at that point, is the terminal); > /tmp/calresult then redirects standard output by itself.
This construct can also be used with piping:
$ cal 17 2009 2>&1 | head -2
This will feed both the standard output and the standard error from cal into the standard input of head .
4.11.3.2. ...redirecting to a device?
Linux treats most devices as files, so you can redirect data to and from devices easily. This command copies the first 50 lines of the /etc/services file directly to a parallel printer port:
$ head -50 /etc/services > /dev/lp0
4.11.3.3. ...splitting a pipe to send data to two destinations?
The tee command will receive data on standard input and write one copy to a file and one copy to standard output. This effectively splits a pipe:
$ cal -y | tee /tmp/thisyear.txt | head -2
To send a copy of the data to the screen, use tee with the device file /dev/tty (the current terminal):
$ cal -y | tee /dev/tty | grep Mo | head -1 >/tmp/dow-header.txt
4.11.3.4. ...piping and redirecting data that is not text?
No assumptions are made about the type of data being piped or redirected; in fact, there are many programs that are designed to work with piped graphics, audio, or video data streams. For example, this pipeline will decode a color JPEG image, scale it to half-size, convert it to grayscale, normalize it, convert it back into a JPEG, save a copy as /tmp/final.jpg , and display the output in a window:
$ djpeg /usr/share/wallpapers/floating-leaves.jpg | pnmscale 0.5 | ppmtopgm | ppmnorm | cjpeg | tee /tmp/final.jpg | display -
4.11.4. Where Can I Learn More?
The manpage for bash
4.12. Writing Simple Scripts
bash command lines can get to be very long, especially when pipes are used. A script is a text file that contains shell commands that may itself be executed as a command, providing an easy way to reuse complex sequences of commands. In fact, bash provides a complete programming language for use in scripts.