Chris Tyler - Fedora Linux

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July 2006

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Error messages are not sent to standard output, so you can still see the messages even when standard output is redirected:

$ cal 17 2009 > month.txt

cal: illegal month value: use 1-12

To redirect error messages, place the file descriptor number ( 2 ) in front of the redirection symbol ( > or >> ):

$ cal 17 2009 2 >errors

$ cat errors

cal: illegal month value: use 1-12

You can redirect both standard output and standard error:

$ cal 17 2009 > month.txt 2 >errors

To redirect the input of a command, use the less-than sign ( < ) followed by the filename containing the data you wish to use as the input:

$ echo " 2^8 " > problem

$ bc < problem

256

bc is a calculator program. The first command places a numeric expression in the file problem ; the second line starts bc , using problem as the input. The output from bc is the solution of the expression: 256 .

Of course, you can redirect both input and output:

$ bc < problem > result

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! ).

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

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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 .