Shell Input and Output Redirection

The shell performs command and parameter substitution before using the Word or Digit parameter except as noted. File name substitution occurs only if the pattern matches a single file and blank interpretation is not performed. There are several cases:

If any character in word is quoted, the delimiter is formed by removing the quote on word. The here document lines will not be expanded. Otherwise, the delimiter is the word itself. If no characters in word are quoted, all lines of the here document will be expanded for parameter expansion, command substitution, and arithmetic expansion.

The shell performs parameter substitution for the redirected data. To prevent the shell from interpreting the \, $, and single quotation mark (') characters and the first character of the Word parameter, precede the characters with a \ character.

If a hyphen (-) is appended to <<, the shell strips all leading tabs from the Word parameter and the document.

the file descriptor 1 is associated with the file specified by the File parameter. The shell associates file descriptor 2 with the file associated with file descriptor 1 (File). If the order of redirections were reversed, file descriptor 2 would be associated with the terminal (assuming file descriptor 1 had previously been) and file descriptor 1 would be associated with the file specified by the File parameter.

If a command is followed by an ampersand (&) and job control is not active, the default standard input for the command is the empty file /dev/null. Otherwise, the environment for the execution of a command contains the file descriptors of the invoking shell as modified by input and output specifications.

Unix command cat is actually short for "catenate," i.e., link together. It accepts multiple filename arguments and copies them to the standard output. But let's pretend, for the moment, that cat and other utilities don't accept filename arguments and accept only standard input. Unix shell lets you redirect standard input so that it comes from a file. The notation command< filename does this; it sets things up so that command takes standard input from a file instead of from a terminal.

For example, if you have a file called fred that contains some text, then cat < fred will print fred's contents out onto your terminal. sort < fred will sort the lines in the fred file and print the result on your terminal (remember: we're pretending that utilities don't take filename arguments).

Similarly, command> filenamecauses the command's standard output to be redirected to the named file. The classic "canonical" example of this is date > now: the date command prints the current date and time on the standard output; the above command saves it in a file called now.

Input and output redirectors can be combined. For example: the cp command is normally used to copy files; if for some reason it didn't exist or was broken, you could use cat in this way:

It is also possible to redirect the output of a command into the standard input of another command instead of a file. The construct that does this is called the pipe, notated as |. A command line that includes two or more commands connected with pipes is called a pipeline.

Pipes are very often used with the less command, which works just like cat except that it prints its output screen by screen, pausing for the user to type SPACE (next screen), RETURN (next line), or other commands. If you're in a directory with a large number of files and you want to see details about them, ls -l | less will give you a detailed listing a screen at a time.

Pipelines can get very complex they can also be combined with other I/O directors. To see a sorted listing of the file freda screen at a time, type sort < fred | more. To print it instead of viewing it on your terminal, type sort < fred | lp.

Here's a more complicated example. The file /etc/passwd stores information about users' accounts on a UNIX system. Each line in the file contains a user's login name, user ID number, encrypted password, home directory, login shell, and other info. The first field of each line is the login name; fields are separated by colons (:).

To get a sorted listing of all users on the system, type:

$ cut -d: -f1 < /etc/passwd | sort

(Actually, you can omit the <, since cut accepts input filename arguments.) The cut command extracts the first field (-f1), where fields are separated by colons (-d:), from the input.

If you want to send the list directly to the printer (instead of your screen), you can extend the pipeline like this:

$ cut -d: -f1 < /etc/passwd | sort | lp

Now you should see how I/O directors and pipelines support the UNIX building block philosophy. The notation is extremely terse and powerful. Just as important, the pipe concept eliminates the need for messy temporary files to store output of commands before it is fed into other commands.

After sufficient practice, you will find yourself routinely typing in powerful command pipelines that do in one line what it would take several commands (and temporary files) in other operating systems to accomplish.

News	See also	Best Shell Books	Recommended Links	Papers, ebooks tutorials	Pipes	Reference
Pipes in Loops	Process Substitution in Shell	Tee		Tips	Humor	Etc

<&`Digit`	Duplicates standard input from the file descriptor specified by the Digit parameter
>&`Digit`	Duplicates standard output in the file descriptor specified by the Digit parameter
<&-	Closes standard input
>&-	Closes standard output
<&p	Moves input from the co-process to standard input
>&p	Moves output to the co-process to standard output

Shell Input and Output Redirection

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