Redirecting Input and Output

CONCEPT: Every program you run from the shell opens three files: Standard input, standard output, and standard error. The files provide the primary means of communications between the programs, and exist for as long as the process runs.

The standard input file provides a way to send data to a process. As a default, the standard input is read from the terminal keyboard.

The standard output provides a means for the program to output data. As a default, the standard output goes to the terminal display screen.

The standard error is where the program reports any errors encountered during execution. By default, the standard error goes to the terminal display.

CONCEPT: A program can be told where to look for input and where to send output, using input/output redirection. Unix uses the "less than" and "greater than" special characters (< and >) to signify input and output redirection, respectively.

Redirecting input

Using the "less-than" sign with a file name like this:
< file1
in a shell command instructs the shell to read input from a file called "file1" instead of from the keyboard.

EXAMPLE:Use standard input redirection to send the contents of the file /etc/passwd to the more command:

more < /etc/passwd

Many Unix commands that will accept a file name as a command line argument, will also accept input from standard input if no file is given on the command line.

EXAMPLE: To see the first ten lines of the /etc/passwd file, the command:

head /etc/passwd
will work just the same as the command:
head < /etc/passwd

Redirecting output

Using the "greater-than" sign with a file name like this:
> file2
causes the shell to place the output from the command in a file called "file2" instead of on the screen. If the file "file2" already exists, the old version will be overwritten.

EXAMPLE: Type the command

ls /tmp > ~/ls.out
to redirect the output of the ls command into a file called "ls.out" in your home directory. Remember that the tilde (~) is Unix shorthand for your home directory. In this command, the ls command will list the contents of the /tmp directory.

Use two "greater-than" signs to append to an existing file. For example:

>> file2
causes the shell to append the output from a command to the end of a file called "file2". If the file "file2" does not already exist, it will be created.

EXAMPLE: In this example, I list the contents of the /tmp directory, and put it in a file called myls. Then, I list the contents of the /etc directory, and append it to the file myls:

ls /tmp > myls
ls /etc >> myls

Redirecting error

Redirecting standard error is a bit trickier, depending on the kind of shell you're using (there's more than one flavor of shell program!). In the POSIX shell and ksh, redirect the standard error with the symbol "2>".

EXAMPLE: Sort the /etc/passwd file, place the results in a file called foo, and trap any errors in a file called err with the command:

sort < /etc/passwd > foo 2> err

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