As
we all know that every thing in Linux is a file. Any program that
runs on a Linux machine has access to something called a €œFile
Descriptor Table. A This table acts as a map providing the process
access to files, directories, unnamed pipes, named pipes, sockets and
kernel-level data structures. This table exists for each process.
There
are 3 standard file descriptors accessible inside the bash shell.
They are
0
- Input - Keyboard (stdin) - Standard input
1
- Output - Screen (stdout) - Standard output
2
- Error - Screen (stderr) -Standard error
The
above three numbers are standard POSIX numbers and also known as file
descriptors. Every Linux command at least open the above streams to
talk with users or other system programs. These 3 are character
devices. Character devices provide a mechanism to send a stream of
characters or bytes. A stream provides sequential access that is, it
provides output in the order in which it was received, this is also
known as a FIFO pipe, which stands for First-In First-Out.
stdin
0 Read input from a file (the default is keyboard)
cat
< File Name
stdout
1 Send data to a file (the default is screen).
date
> Output File Name
cat
Output File Name
stderr
2 Send all error messages to a file (the default is screen).
rm
/tmp/File Name 2> error.txt
cat
Error.txt
Using
the Above FD Numbers, 2> redirects file descriptor 2, or standard
error. &n is the syntax for redirecting to a specific open file
(Because 0 (stdin), 1 (stdout) and 2 (stderr) are actually file
descriptors the shell requires an ampersand put in front of them for
redirection. It duplicates the file descriptor in this case
effectively merging the two streams of information together. if you
just had "1" with no ampersand, the shell would create a
file named "1" and redirect stderr output to it).
Now
when you try to execute the below command which gives an error like,
Dev:vx1423:djbs002-jas
$ ls -l myfile.txt > test.txt
ls:
myfile.txt: No such file or directory
We
can redirect the error to a file like
Dev:vx1423:djbs002-jas
$ ls -l myfile.txt 2> test.txt
Check
the Contents of the Error File
Dev:vx1423:djbs002-jas
$ cat test.txt
ls:
myfile.txt: No such file or directory
For
example 2>&1 redirects 2 (standard error) to 1 (standard
output); if 1 has been redirected to a file, 2 goes there too.
Character
Action
> Redirect
standard output
2> Redirect
standard error
2>&1 Redirect
standard error to standard output
< Redirect
standard input
| Pipe
standard output to another command
>> Append
to standard output
2>&1| Pipe
standard output and standard error to another command
/dev/null
& /dev/zero
All
data written on a /dev/null or /dev/zero special file is discarded by
the system. Use /dev/null to send any unwanted output from
program/command
command
>/dev/null
This
syntax redirects the command standard output messages to /dev/null
where it is ignored by the shell.
command
2>/dev/null
This
syntax redirects the command error output messages to /dev/null where
it is ignored by the shell.
command
&>/dev/null
This
syntax redirects both standard output and error output messages to
/dev/null where it is ignored by the shell.
As
an Example
Localhost:Root$
grep root /etc/passwd && echo "Found" || "Not
Found"
root:x:7282:1566:Nohting:/privdir/root:/bin/bash
Found
This
Command shows the Standard Output as well as the Result. If you need
only the result ,then send the output to /dev/null like
Localhost:Root
$ grep root /etc/passwd > /dev/null && echo "Found"
|| "Not Found"
Found
A
Standard Error Re-direction include
command-name
2>error.log
LocalHost:Root$
find /home -name .profile 2>/tmp/error
LocalHost:Root
$ cat /tmp/error
find:
/home/lost+found: Permission denied
find:
/home/dsig999: Permission denied
Redirect
Script Errors
You
can redirect script error to a log file called scripts.err:
./script.sh
2>scripts.err
/path/to/example.pl
2>scripts.err
Append
To Error Log
You
can append standard error to end of error.log file using >>
operator:
command-name
2>>error.log
./script.sh
2>>error.log
/path/to/example.pl
2>>error.log
2>&1
The
1 denotes standard output (stdout). The 2 denotes standard error
(stderr).
command-name
>/dev/null 2>&1
So
2>&1 says to send standard error to where ever standard output
is being redirected as well.
Which
since it's being sent to /dev/null is akin to ignoring any output at
all.
Input
redirection can be useful if you have written a program which expects
input from the terminal and you want to provide it from a file.
$
myprog < Input File > Output File
To
redirect standard error and output to different files (note that
grouping is not necessary in Bourne shell):
$
cat myfile > Output File 2> Error File
Assigns
the file descriptor (fd) to file for output
Besides
1,2 and 3 there are also file Descriptors from 3 to 1023 which are
free to use. For each one a symlink in /dev/fd is created as soon as
it is initialized.
For
creating a new FD , we need to use the “exec” command.
The
internal exec command replaces the current shell process with the
specified command. Normally, when you run a command a new process is
spawned. The exec command does not spawn a new process. Instead, the
current process is overlaid with the new command.
You
can assign a file descriptor to an output file with the following
syntax:
exec
fd> output.txt
where,
fd >= 3
Dev:vx1423:djbs002-jas
$ exec 4> /config/jboss/ews/1.0/domains/jas/noter
Dev:vx1423:djbs002-jas
$ echo "This is a test" >&4
Dev:vx1423:djbs002-jas
$ cat noter
This
is a test
exec
4<&-
Assigns
the file descriptor (fd) to file for input
exec
fd< input.txt
exec
3< /etc/resolv.conf
#
Executes cat commands and read input from
#
the file descriptor (fd) # 3 i.e. read input from /etc/resolv.conf
file
cat
<&3
#
Close fd # 3
exec
3<&-
To
Create a read/Write FD for a File use,
exec
3<> file
You
can use exec to perform I/O redirection commands. The following
examples illustrate the use of exec for redirection purposes.
exec
3< inputfile # Opens inputfile with file descriptor 3 for reading.
exec
4> outputfile # Opens outputfile with file descriptor 4 for
writing.
exec
5<&0 # Makes fd 5 a copy of fd 0 (standard input).
exec
6>&p # Attach fd 6 to co-process.
