Riccardo Riva

This would be another little memo of all the most used (by me) Input/output redirections and pipes.
As other times this would not be a complete guide, and would not go into the deep.

The base :

Standard input (stdin) : 0
Standard output (stdout) : 1
Error output (stderr) : 2

> : output redirection
< : input redirection
>> : append output
| : pass the output to the next utility
|| : execute the next command if the previous failed
&& : execute the next command only if the previous has succeded

Examples :

cat file.txt > /tmp/file.txt

If file.txt exists, the output will be written in /tmp/file.txt
If file.txt doesn’t exist, the error message will be output to the screen while nothing will be written to /tmp/file.txt (but the file will be created if not existing)

cat file.txt 2> /tmp/error_file.txt > /tmp/file.txt

In this case the error message would be written to /tmp/error_file.txt if the file doesn’t exist

cat file.txt 2> /tmp/error_file.txt >> /tmp/file.txt

The error output would still go in error_file.txt while the standard output would be APPENDED to file.txt

cat file1 > file2

Will copy the content of file1 to file2

 cat > file3

Hello world

Will write “Hello world” to file3

[/bash]wc < file.txt > count_file.txt[/bash]

This would count the number of lines, words and bytes in file.txt (”wc < file.txt”) and output the result to count_file.txt

Several ways of counting lines etc. :
$ wc file.txt
7 2 17 file.txt
$ wc < file.txt
7 2 17
$ cat file.txt | wc
7 2 17

ls -l test10 > file.txt 2>&1

This would output anything under file.txt

<code>ls -l test10 2>&1 | mail -s “output in a mail” address@domain.com
Any output in a mail

Hope this help

Bye
Riccardo

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