Working with Standard Input, Output & Error in Linux

Every process in Linux is provided with three open files( usually called file descriptor). These files are the standard input, output and error files. By default :

  • Standard Input is the keyboard, abstracted as a file to make it easier to write shell scripts.
  • Standard Output is the shell window or the terminal from which the script runs, abstracted as a file to again make writing scripts & program easier
  • Standard error is the same as standard output:the shell window or terminal from which the script runs.

A file descriptor is simply a number that refers to an open file. By default , file descriptor 0 (zero) refers to the standard input & often abbreviated as stdin. File descriptor 1 refers to standard output (stdout) and file descriptor 2 refers to standard error (stderr). These numbers are important when you need to access a particular file , especially when you want to redirect these files to the other locations, File descriptors numbers go up from zero.

Redirecting Standard Output

Syntax to redirect the output of a command to a file.

# Command_options_and_arguments > output_file

Example :

linuxtechi@localhost:~$ cat /proc/cpuinfo > command.txt

We can see the data that would have gone to the screen with more command :

linuxtechi@localhost:~$ more command.txt 
processor         : 0
vendor_id         : GenuineIntel
cpu family        : 6
model             : 37
model name        : Intel(R) Core(TM) i3 CPU       M 370  @ 2.40GHz
stepping          : 5
microcode         : 0x616
cpu MHz           : 0.000
cache size        : 6144 KB
physical id       : 0
siblings          : 2
core id           : 0
cpu cores         : 2
apicid            : 0
initial apicid    : 0
fpu               : yes
fpu_exception     : yes
cpuid level       : 5
wp                : yes

The > operator tells the shell to redirect the output of the command to the given file. If the file exists , the deletes the old contents of the file and replaces it with the output of the command.

Redirecting a Command’s Input :

Syntax to redirect the input of a command to come from a file.

# Command_options_and_arguments < input_file

Use the < operator to redirect the input for a command , example is shown below :

linuxtechi@localhost:~$ wc -l < command.txt
52

In this example , the input to the ‘wc‘ command comes from the file named command.txt. The shell sends the contents of the file command.txt as a standard input for the wc command.

Note : We can also combine both redirections with following syntax :

# command_options_and_agruments < input_file > output_file.

Redirecting Standard Error :

In addition to redirecting the standard input and output for a script or a command, we can also redirect standard error. Even though standard error by defaults goes to the same place as the standard output – the shell window or terminal. There are good reasons why stdout and stderr are treated separately. The main reason is that we can redirect the output of a command or commands to a file but you have no way of knowing whether an error occurred. Separating stderr from stdout allows the error message to appear on your screen while output still goes to a file.

Syntax to redirect stderr from a command to a file.

# command_options_and_agruments 2> output_file.

The 2 in 2> refers to the file descriptor 2, the descriptor number for stderr.

Example:

linuxtechi@localhost:~$ lsash /usr/bin 2> commands-error.txt
linuxtechi@localhost:~$ cat commands-error.txt
No command 'lsash' found, did you mean:
Command 'sash' from package 'sash' (universe)
lsash: command not found

Redirecting both Standard Ouput & Standard Error.

Use 2>&1 Syntax to redirect standard error to the same location as standard output .

Example:1

linuxtechi@localhost:~$ ls /usr/bin > command.txt 2>&1

Above Command has three parts.

  • ls /usr/bin is the command run
  • > command.txt redirects the output of the ls command
  • 2>&1 sends the output of the file descriptor 2, stderr , to the same location as the file descriptor 1, stdout.

Example: 2

linuxtechi@localhost:~$ ls /usr2222/bin > command.txt 2>&1
linuxtechi@localhost:~$ more command.txt
ls: cannot access /usr2222/bin: No such file or directory

Note that above example assumes that your system doesn’t have directory names “/usr2222/bin”

Redirecting Both stderr & stdout at Once

linuxtechi@localhost:~$ ls /usr2222/bin &> command.txt
linuxtechi@localhost:~$ more command.txt
ls: cannot access /usr2222/bin: No such file or directory

In the above command ls is the command , /usr2222/bin is the argument to the ‘ls‘ command and ‘&> command.txt‘ redirect both stdout and stderr to a file named command.txt.

Appending To Files

Use the ‘>>’ operator to redirect the output of a command , but append to the file , if it exists. The syntax is given below :

# Command >> file_to_append.

Example:

linuxtechi@localhost:~$ uptime >> sysload.txt
linuxtechi@localhost:~$ uptime >> sysload.txt
linuxtechi@localhost:~$ uptime >> sysload.txt
linuxtechi@localhost:~$ more sysload.txt
11:49:17 up 1:22, 3 users, load average: 0.28, 0.12, 0.11
11:49:28 up 1:22, 3 users, load average: 0.40, 0.15, 0.12
11:49:36 up 1:23, 3 users, load average: 0.33, 0.14, 0.12

Truncating Files :

We can use a shorthand syntax for truncating files by omitting the command before > operator . The Syntax is given below :

# > file_name

We can also use an alternate format with a colon :

# : > file_name

Both of these command-less command will create the file if it does not exist and truncate the file to zero bytes if the file does exist.

linuxtechi@localhost:~$ ls /usr/bin > command.txt
linuxtechi@localhost:~$ ls -l command.txt
-rw-rw-r-- 1 linuxtechi linuxtechi 19713 Dec 2 12:18 command.txt
linuxtechi@localhost:~$ > command.txt
linuxtechi@localhost:~$ ls -l command.txt
-rw-rw-r-- 1 linuxtechi linuxtechi 0 Dec 2 12:18 command.txt

Sending Ouput to Nowhere Fast

There are some scenarios where you not only want to redirect the output of a command , you want to throw the output away. You can do this by redirecting a command’s output to the null file “/dev/null” The null file consumes all output sent to it , as if /dev/null is a black hole star.

linuxtechi@localhost:~$ ls /usr/bin > /dev/null

Note : The file /dev/null is often called a bit bucket.

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