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Created page with "== Standard File Descriptors == On Linux, other Unix-like systems, and on Windows, programs may open file descriptors (Linux terminology) or handles (Windows terminology). Ea..."
== Standard File Descriptors ==
On Linux, other Unix-like systems, and on Windows, programs may open file descriptors (Linux terminology) or handles (Windows terminology). Each descriptor/handle is a numbered channel connected to a file or device.
By default, three channels are opened automatically by the shell when a process is started. These are:
0 - Standard Input (stdin) - this is the default input channel for the program
1 - Standard Output (stdout) - this is the default output channel for the program, used to output "normal" messages
2 - Standard Error (stderr) - this is the default error channel for the program, used to output error messages
Without redirection, all three of these descriptors/handles are connected to the terminal. Therefore, the command will get input from the terminal, send output messages to the terminal, and send error messages to the terminal.
== File Redirection ==
Any of these descriptors/handles may be connected to a different file or device by adding symbols to the command line.
These are the most commonly-used symbols:
>file redirects stdout to the specified file
>>file redirects stdout to the specified file, appending
<file redirects stdin from the specified file
Both the > and >> symbols will create the file if it does not exist. If the file does exist, the > symbol will overwrite it, while the >> symbol will append to it (add to the end of the file).
Examples on Linux:
$ '''date >now''' # redirect the output of the date command into the file named "now"
$ '''cat now''' # display the file contents
Tue 26 Sep 2028 01:14:02 PM EDT
$ '''date >>now''' # append the output of the date command into the file "now"
$ '''cat now''' # display the file contents - note that there are two dates
Tue 26 Sep 2028 01:14:02 PM EDT
Tue 26 Sep 2028 01:14:10 PM EDT
$ '''date >>now''' # repeat a third time
$ '''cat now'''
Tue 26 Sep 2028 01:14:02 PM EDT
Tue 26 Sep 2028 01:14:10 PM EDT
Tue 26 Sep 2028 01:14:22 PM EDT
$ '''date >now''' # redirect with a single > character - will overwrite
$ '''cat now''' # display the file contents - note the old data was overwritten
Tue 26 Sep 2028 01:14:28 PM EDT
The same example on Windows:
> date /t >now.txt
> type now.txt
2028-09-26
> date /t >>now.txt
> type now.txt
2028-09-26
2028-09-26
> date /t >>now.txt
> type now.txt
2028-09-26
2028-09-26
2028-09-26
> date /t >now.txt
> type now.txt
2023-09-26
To redirect a different file descriptior/handle, place the descriptor/handle number in front of the redirection symbol:
2>file redirects stderr (2) to the specified file
2>>file redirects stderr (2) to the specified file, appending
Examples on Linux:
$ '''touch one two''' # create the files "one" and "two"
$ '''rm three''' # make sure that no file named "three" exists
rm: cannot remove 'three': No such file or directory
$ '''ls -l one two three''' # this should succeed for 2 files, fail for 1 file
ls: cannot access 'three': No such file or directory
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
$ '''ls -l one two three''' >listing.txt # redirect output but not errors
ls: cannot access 'three': No such file or directory
$ '''cat listing.txt''' # view the saved output
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
$ '''ls -l one two three''' >listing.txt 2>errors.txt # output and errors redirected separately
$ '''cat listing.txt''' # view the saved output
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
$ '''cat errors.txt''' # view the saved error messages
ls: cannot access 'three': No such file or directory
To redirect one descriptor/handle to another descriptor/handle, use the syntax:
X>&Y
Where X is the descriptor/handle you're redirecting, and Y is the target descriptor/handle.
For example, on Linux:
$ '''ls -l one two three >all.txt 2>&1''' # redirect stdout to all.txt, then redirect stderr to the same place as stdout
$ '''cat all.txt''' # view the contents of all.txt
ls: cannot access 'three': No such file or directory
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
On Linux, other Unix-like systems, and on Windows, programs may open file descriptors (Linux terminology) or handles (Windows terminology). Each descriptor/handle is a numbered channel connected to a file or device.
By default, three channels are opened automatically by the shell when a process is started. These are:
0 - Standard Input (stdin) - this is the default input channel for the program
1 - Standard Output (stdout) - this is the default output channel for the program, used to output "normal" messages
2 - Standard Error (stderr) - this is the default error channel for the program, used to output error messages
Without redirection, all three of these descriptors/handles are connected to the terminal. Therefore, the command will get input from the terminal, send output messages to the terminal, and send error messages to the terminal.
== File Redirection ==
Any of these descriptors/handles may be connected to a different file or device by adding symbols to the command line.
These are the most commonly-used symbols:
>file redirects stdout to the specified file
>>file redirects stdout to the specified file, appending
<file redirects stdin from the specified file
Both the > and >> symbols will create the file if it does not exist. If the file does exist, the > symbol will overwrite it, while the >> symbol will append to it (add to the end of the file).
Examples on Linux:
$ '''date >now''' # redirect the output of the date command into the file named "now"
$ '''cat now''' # display the file contents
Tue 26 Sep 2028 01:14:02 PM EDT
$ '''date >>now''' # append the output of the date command into the file "now"
$ '''cat now''' # display the file contents - note that there are two dates
Tue 26 Sep 2028 01:14:02 PM EDT
Tue 26 Sep 2028 01:14:10 PM EDT
$ '''date >>now''' # repeat a third time
$ '''cat now'''
Tue 26 Sep 2028 01:14:02 PM EDT
Tue 26 Sep 2028 01:14:10 PM EDT
Tue 26 Sep 2028 01:14:22 PM EDT
$ '''date >now''' # redirect with a single > character - will overwrite
$ '''cat now''' # display the file contents - note the old data was overwritten
Tue 26 Sep 2028 01:14:28 PM EDT
The same example on Windows:
> date /t >now.txt
> type now.txt
2028-09-26
> date /t >>now.txt
> type now.txt
2028-09-26
2028-09-26
> date /t >>now.txt
> type now.txt
2028-09-26
2028-09-26
2028-09-26
> date /t >now.txt
> type now.txt
2023-09-26
To redirect a different file descriptior/handle, place the descriptor/handle number in front of the redirection symbol:
2>file redirects stderr (2) to the specified file
2>>file redirects stderr (2) to the specified file, appending
Examples on Linux:
$ '''touch one two''' # create the files "one" and "two"
$ '''rm three''' # make sure that no file named "three" exists
rm: cannot remove 'three': No such file or directory
$ '''ls -l one two three''' # this should succeed for 2 files, fail for 1 file
ls: cannot access 'three': No such file or directory
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
$ '''ls -l one two three''' >listing.txt # redirect output but not errors
ls: cannot access 'three': No such file or directory
$ '''cat listing.txt''' # view the saved output
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
$ '''ls -l one two three''' >listing.txt 2>errors.txt # output and errors redirected separately
$ '''cat listing.txt''' # view the saved output
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
$ '''cat errors.txt''' # view the saved error messages
ls: cannot access 'three': No such file or directory
To redirect one descriptor/handle to another descriptor/handle, use the syntax:
X>&Y
Where X is the descriptor/handle you're redirecting, and Y is the target descriptor/handle.
For example, on Linux:
$ '''ls -l one two three >all.txt 2>&1''' # redirect stdout to all.txt, then redirect stderr to the same place as stdout
$ '''cat all.txt''' # view the contents of all.txt
ls: cannot access 'three': No such file or directory
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
