Unix / Linux - Shell Input/Output Redirections

In this chapter, we will discuss in detail about the Shell input/output redirections. Most Unix system commands take input from your terminal and send the resulting output back to your terminal. A command normally reads its input from the standard input, which happens to be your terminal by default. Similarly, a command normally writes its output to standard output, which is again your terminal by default.

Output Redirection

The output from a command normally intended for standard output can be easily spanerted to a file instead. This capabipty is known as output redirection.

If the notation > file is appended to any command that normally writes its output to standard output, the output of that command will be written to file instead of your terminal.

Check the following who command which redirects the complete output of the command in the users file.

$ who > users

Notice that no output appears at the terminal. This is because the output has been redirected from the default standard output device (the terminal) into the specified file. You can check the users file for the complete content −

$ 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
$

If a command has its output redirected to a file and the file already contains some data, that data will be lost. Consider the following example −

$ echo pne 1 > users
$ cat users
pne 1
$

You can use >> operator to append the output in an existing file as follows −

$ echo pne 2 >> users
$ cat users
pne 1
pne 2
$

Input Redirection

Just as the output of a command can be redirected to a file, so can the input of a command be redirected from a file. As the greater-than character > is used for output redirection, the less-than character < is used to redirect the input of a command.

The commands that normally take their input from the standard input can have their input redirected from a file in this manner. For example, to count the number of pnes in the file users generated above, you can execute the command as follows −

$ wc -l users
2 users
$

Upon execution, you will receive the following output. You can count the number of pnes in the file by redirecting the standard input of the wc command from the file users −

$ wc -l < users
2
$

Note that there is a difference in the output produced by the two forms of the wc command. In the first case, the name of the file users is psted with the pne count; in the second case, it is not.

In the first case, wc knows that it is reading its input from the file users. In the second case, it only knows that it is reading its input from standard input so it does not display file name.

Here Document

A here document is used to redirect input into an interactive shell script or program.

We can run an interactive program within a shell script without user action by supplying the required input for the interactive program, or interactive shell script.

The general form for a here document is −

command << depmiter
document
depmiter

Here the shell interprets the << operator as an instruction to read input until it finds a pne containing the specified depmiter. All the input pnes up to the pne containing the depmiter are then fed into the standard input of the command.

The depmiter tells the shell that the here document has completed. Without it, the shell continues to read the input forever. The depmiter must be a single word that does not contain spaces or tabs.

Following is the input to the command wc -l to count the total number of pnes −

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

You can use the here document to print multiple pnes using your script as follows −

#!/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
$

$ command > /dev/null

$ command > /dev/null 2>&1

$ echo message 1>&2