Output Redirection:
The output from a
command normally intended for standard output can be easily can be diverted to a file
instead. This capability is known as output redirection: this is useful to
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 following who command which would redirect
complete output of the command in 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. If you would check users file
then it would have complete content:
$
cat users
root :0 Nov 7 12:11
root pts/0 Nov 7 12:12 (:0.0)
root pts/1 Nov 7 12:20 (:0.0)
$
If a command has its
output redirected to a file and the file already contains some data, that data
will be lost. Consider this example:
$
echo line 1 > users
$
cat users
line
1
$
You can use >>
operator to append the output in an existing file as follows:
$
echo line 2 >> users
$
cat users
line
1
line
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 standard input
can have their input redirected from a file in this manner. For example, to
count the number of lines in the file users generated above,
you can execute the command as follows:
$
wc -l
users
2 users
$
Here it produces output 2 lines. You can count the number of lines
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 listed with the line 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
<<
delimiter
document
delimiter
Here the shell
interprets the << operator as an instruction to read input until it finds
a line containing the specified delimiter. All the input lines up to the line
containing the delimiter are then fed into the standard input of the command.
The delimiter tells the
shell that the here document has completed. Without it, the shell continues to
read input forever. The delimiter must be a single word that does not contain
spaces or tabs.
Following is the input to the command wc -l to
count total number of line:
$wc
-l
<<
EOF
This is a simple lookup program
for good (and bad) restaurants
in Cape Town.
EOF
3
$
You can use here document to print multiple lines
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 would produce
following result:
This is a simple lookup program
for good (and bad) restaurants
in Cape Town.
The following script
runs a session with the vi text editor and save the input in the file test.txt.
#!/bin/sh
filename=test.txt
vi
$filename <<EndOfCommands
i
This file was created automatically from
a
shell script
^[
ZZ
EndOfCommands
If you run this script
with vim acting as vi, then you will likely see output like the following:
$
sh test.sh
Vim: Warning: Input is not from a terminal
$
After running the
script, you should see the following added to the file test.txt:
$
cat test.txt
This file was created automatically from
a
shell script
$
Discard the output:
Sometimes you will need
to execute a command, but you don't want the output displayed to the screen. In
such cases you can discard the output by redirecting it to the file /dev/null:
$
command > /dev/null
Here command is the name
of the command you want to execute. The file /dev/null is a special file that
automatically discards all its input.
To discard both output
of a command and its error output, use standard redirection to redirect STDERR
to STDOUT:
$
command > /dev/null 2>&1
Here 2 represents STDERR
and 1 represents STDOUT. You can display a message on to STDERR by redirecting
STDIN into STDERR as follows:
$
echo message 1>&2
Redirection Commands:
Following is the
complete list of commands which you can use for redirection:
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.
|
Note that file
descriptor 0 is normally standard input (STDIN), 1 is standard output (STDOUT),
and 2 is standard error output (STDERR).
