UNIX TUTORIALS - Unix - IO Redirections

Unix - IO Redirections

Redirection Commands:

Command	Description
pgm > file	Output of pgm is redirected to file
pgm < file	Program pgm reads its input from file.
pgm >> file	Output of pgm is appended to file.
n > file	Output from stream with descriptor n redirected to file.
n >> file	Output from stream with descriptor n appended to file.
n >& m	Merge output from stream n with stream m.
n <& m	Merge input from stream n with stream m.
<< tag	Standard input comes from here through next tag at start of line.
\|	Takes output from one program, or process, and sends it to another.

Output Redirection:

$ who > users

ADVERTISEMENTS

$ cat users oko tty01 Sep 12 07:30 ai tty15 Sep 12 13:32 ruth tty21 Sep 12 10:10 pat tty24 Sep 12 13:07 steve tty25 Sep 12 13:03 $

$ echo line 1 > users
$ cat users
line 1
$

						
$ echo line 2 >> users
$ cat users
line 1
line 2
$

						Input Redirection:

$ wc -l users
2 users
$

						
$ wc -l < users
2
$

						Here Document:

command << delimiter
document
delimiter

						
$wc -l << EOF
	This is a simple lookup program 
	for good (and bad) restaurants
	in Cape Town.
EOF
3
$

						
#!/bin/sh

cat << EOF
This is a simple lookup program 
for good (and bad) restaurants
in Cape Town.
EOF	

						
This is a simple lookup program
for good (and bad) restaurants
in Cape Town.

						
#!/bin/sh

filename=test.txt
vi $filename <<EndOfCommands
i
This file was created automatically from
a shell script
^[
ZZ
EndOfCommands

						
$ sh test.sh
Vim: Warning: Input is not from a terminal
$

						
$ cat test.txt
This file was created automatically from
a shell script
$

						Discard the output:

$ command > /dev/null

						
$ command > /dev/null 2>&1

						
$ echo message 1>&2