I/O Redirection

See also

• I/O Redirection: Swap stdout And stderr

Standard I/O Streams

• Three Standard I/O file descriptors

• By default bound to the terminal ⟶ keyboard and screen

Meaning	Name	Descriptor	C Macro
Standard input	stdin	0	STDIN_FILENO
Standard output	stdout	1	STDOUT_FILENO
Standard error	stderr	2	STDERR_FILENO

Important

A tradition which is religiously adhered to in Unix

• Debug and error output goes to standard error ⟶ can be redirected to logfile, for example (or discarded into /dev/null)

• Data goes to standard output ⟶ can be fed into another programm via the pipe

I/O Redirection Operators

command < file	command is fed file contents as standard input (i.e. as if typed on keyboard)
command > file	command writes standard output into file (not on screen)
command 2> file	command writes standard error into file (not on screen)

Variation: non-destructive redirection

• > truncates file prior to writing, if it exists

• >> appends to file (or creates if it does not exist)

Example: Standard Output Redirection

Documentation

• man -s 1 grep

Say we are using the grep program to search /etc/passwd for lines containing the word jfasch. It writes the lines (only one in out case) that it finds to standard output:

$ grep jfasch /etc/passwd
jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash

Redirect its output into a file that we might keep for later use,

$ grep jfasch /etc/passwd > found.csv
$ ls -l found.csv
-rw-rw-r--. 1 jfasch jfasch 62 May 12 13:28 found.csv
$ cat found.csv
jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash

Example: Standard Input Redirection

Although grep can open its input file by itself when given as a second commandline parameter, it reads from standard input when not.

$ grep jfasch < /etc/passwd
jfasch:x:1000:1000:Joerg Faschingbauer:/home/jfasch:/bin/bash

Example: Standard Error Redirection

grep emits error messages on standard error in some cases, on of which is where it cannot find the input file given,

$ grep jfasch /etc/passwd-does-not-exist
grep: /etc/passwd-does-not-exist: No such file or directory

If, for example, we want to keep error messages for later debugging (a larger shell script, say), we can redirect standard error to a file. Remember, standard error is on file descriptor 2.

$ grep jfasch /etc/passwd-does-not-exist 2>errors.lst
$ ls -l errors.lst
-rw-rw-r--. 1 jfasch jfasch 60 May 12 14:21 errors.lst
$ cat errors.lst
grep: /etc/passwd-does-not-exist: No such file or directory

Another way of treating error is to ignore them. In this case, the character special device /dev/null is usually used (it behaves like a filefor the purpose of redirection):

$ grep jfasch /etc/passwd-does-not-exist 2>/dev/null

Example: Everything Redirection

If we want (not to say that we have to though!), we can combine redirections of all three standard streams in a way that grep

• Reads from standard input

• Writes standard output to a file

• Write standard error to /dev/null

$ grep jfasch < /etc/passwd > found.csv 2>/dev/null

Important

When multiple redirections are used, the operations are executed from left to right!

This might not sound so important from what we’ve learned so far - but see, for example, I/O Redirection: Swap stdout And stderr.