Linux Bash Scripting: Standard file descriptors STDIN, STDOUT and STDERR and input/ouput redirection

 

Linux system handles every object as a file. This includes input and output processes also. The linux identifies each file using a file descriptor.

What is a file descriptor?
A file descriptor is a non negative integer which uniquely identifies the open files. Each process can have upto 9 open files at a time. The bash shell reserved three file descriptors(0, 1 and 2) for special purposes.

Linux Standard File Descriptors
File descriptor	Abbreviations	Description
0	STDIN	Standard Input
1	STDOUT	Standard output
2	STDERR	Standard Error

The above three file descriptor controls the input and output of your scripts. The shell uses these descriptors to assign the input and output to appropriate locations (default is the monitor or the terminal).  Now let us see each one in detail.

STDIN

The STDIN file descriptor refers the keyboard in the terminal environment. So, the Linux will read the input from keyboard always unless it is changed.

$ cat This is test This is second test  

The above “cat” command accepts input from the keyboard.  If you want to force the “cat” command to accept the input other than STDIN, you can use STDIN redirect symbol (<).

$ cat < filetest This is test This is second test $

STDOUT

The STDOUT descriptor refers to the standard output of the shell (i.e. the monitor). So, the Linux directs all the output to the monitor unless it is changed.

$ ls –l > test $ cat test -rwxrwxrwx 1 sittik owner 260 Oct 16 07:03 arr.sh -rwx--x--x 1 sittik owner 963 Oct 11 01:25 bigchess.sh -rwx--x--x 1 sittik owner 389 Oct 4 00:47 chess.sh

You can also append the data to a file by using the >> symbol.

$ who >> test $ cat test -rwxrwxrwx 1 sittik owner 260 Oct 16 07:03 arr.sh -rwx--x--x 1 sittik owner 963 Oct 11 01:25 bigchess.sh -rwx--x--x 1 sittik owner 389 Oct 4 00:47 chess.sh sittik pts/0 2012-10-20 05:50 (132.127.131.35)

But whenever the command produces an error, it will be displayed in the monitor even if the output is redirected to a file. That is because the error details have a separate file descriptor (STDERR).

STDERR

By default the STDERR descriptor refers to monitor.

$ ls –al badfile > test ls: badfile: No such file or directory $ cat test $

The above example shows that the error message is displayed even after the output is redirected to the file called test. If you want to redirect the error message to some other file, you can do it as below.

$ ls –al badfile 2> test $ cat test ls: badfile: No such file or directory $

If you want to redirect the output and the error message to different files, you can do as below.

$ ls –al goodfile badfile 2> badtest 1> goodtest $ cat badtest ls: badfile: No such file or directory $ cat goodtest -rwx--x--x 1 sittik owner 389 Oct 14 10:47 goodfile $

If you want to redirect both the error message and the output to the same file, you can use the & operator as shown below.

$ ls –al badfile goodfile &> commonfile $ cat commonfile ls: badfile: No such file or directory -rwx--x--x 1 sittik owner 389 Oct 14 10:47 goodfile $