Standard File Descriptors
Linux systems use file descriptors to keep track of open files. Normally, three file descriptors are automatically opened for a process (running instance of a program):
|File Descriptor Number||Name||Short Name||Purpose|
|0||Standard Input||stdin||The normal input to the process.|
|1||Standard Output||stdout||The normal output from the process.|
|2||Standard Error||stderr||Error message output from the process.|
When running commands from the command line, all three of these file descriptors is connected to the terminal, so the input to process comes from the terminal (keyboard), and normal output as well as error output goes to the terminal (screen).
The meaning of these file descriptors may vary in some contexts; for example, when a program is executed as a CGI script by a webserver, the CGI standard dictates that stdin receives POSTed data from the browser, and stdout is used to send the resulting data (typically HTML) from the script. Stderr is usually sent to a logfile.
Additional file descriptors may be opened by a process.
Redirecting the Standard File Descriptors
BASH (and other shells in the Bourne family) permit input and output to be redirected, so that it does not come from or go to the terminal.
In BASH it is possible to redirect output using the
>> symbols after a command and arguments, followed by the filename which will contain the redirected output. These symbols may be preceeded by a file descriptor number; if omitted, stdout is assumed. The
> symbol will truncate the file if it already exists;
>> will append to the existing file. Both symbols will create the file if it does not exist.
cal command (which outputs a calendar of the current month by default) and places the output in the file "calendar.txt". If the file "calendar.txt" already exists, it is truncated (the existing contents are wiped out). If it does not exist, it is created.
cal 3 2025 >>calendar.txt
Appends to the calendar for March, 2008 to the file "calendar.txt". The file is created if it does not exist.
cal 16 2009 >output 2>error
This command will fail because there is no 16th month in the year 2009 (or any year!). The stdout is redirected to the file "output", but there will be no output. The stderr (error message) will be redirected to the file "calendar.txt".
BASH uses the
<< symbols to redirect input. The
< symbol used after a command and arguments and is followed by the filename from which input will be taken. If the input file does not exist, an error message is displayed. For example:
mail -s "Suggestion" firstname.lastname@example.org <draft3.txt
This command will mail the contents of the file "draft3.txt" to email@example.com with a subject line of "Suggestion".
<< symbol is only used within scripts. It is placed after a command and arguments and is followed by a text label. The lines after the command up to (but not including) the line containing the text label are used as the stdin of the command. For example:
mail -s "Warning" firstname.lastname@example.org <<EOF This is a warning that the disk space on your system has fallen below 1 GB. It is recommended that you delete some files to free up additional disk space. EOF
This will use the three lines after the