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i/o redirection

There are always three default files open, STDIN, STDOUT, and STDERR. These, and any other open files, can be redirected. Redirection simply means capturing output from a file, command, program, script, or even code block within a script and sending it as input to another file, command, program, or script.

Name Default Destination Redirection Use File Descriptor Number
STDIN Keyboard < 0
STDOUT Monitor > 1
STDERR Monitor 2> 2

Using I/O Redirection

Redirector Explanation Example
1> Redirect STDOUT 1> stdout.txt
1>> Redirect and append STDOUT 1>> stdout.txt
2> Redirect STDERR 2> stderr.txt
2>> Redirect and append STDERR 2>> stderr.txt
&> Redirect both STDOUT and STDERR &> stdout_err.txt
&>> Redirect and append both STDOUT and STDERR &>> stdout_err.txt

Using Pipes
A pipe | can be used to catch the output of one command and use that as input for a second command. A pipe | can be useful for chaining commands, scripts, files, and programs together.


 cat *.txt | sort | uniq > result-file
 # Sorts the output of all the .txt files and deletes duplicate lines,
 # finally saves results to "result-file".


Advanced Bash-Scripting Guide: Chapter 20. I/O Redirection