Standard Input and Output Redirection

The shell and many UNIX commands take their input from standard input (stdin), write output to standard output (stdout), and write error output to standard error (stderr). By default, standard input is connected to the terminal keyboard and standard output and error to the terminal screen.

The way of indicating an end-of-file on the default standard input, a terminal, is usually <Ctrl-d>.

Redirection of I/O, for example to a file, is accomplished by specifying the destination on the command line using a redirection metacharacter followed by the desired destination.

C Shell Family

Some of the forms of redirection for the C shell family are:

The form of a command with standard input and output redirection is:

% command -[options] [arguments] < input file  > output file 

If you are using csh and do not have the noclobber variable set, using > and >& to redirect output will overwrite any existing file of that name. Setting noclobber prevents this. Using >! and >&! always forces the file to be overwritten. Use >> and >>& to append output to existing files.

Redirection may fail under some circumstances: 1) if you have the variable noclobber set and you attempt to redirect output to an existing file without forcing an overwrite, 2) if you redirect output to a file you don't have write access to, and 3) if you redirect output to a directory.

Examples:

% who > names

Redirect standard output to a file named names

% (pwd; ls -l) > out

Redirect output of both commands to a file named out

% pwd; ls -l > out

Redirect output of ls command only to a file named out

Input redirection can be useful, for example, if you have written a FORTRAN program which expects input from the terminal but you want it to read from a file. In the following example, myprog, which was written to read standard input and write standard output, is redirected to read myin and write myout:

% myprog < myin > myout 

You can suppress redirected output and/or errors by sending it to the null device, /dev/null. The example shows redirection of both output and errors:

% who >& /dev/null 

To redirect standard error and output to different files, you can use grouping:

% (cat myfile > myout) >& myerror 

Bourne Shell Family

The Bourne shell uses a different format for redirection which includes numbers. The numbers refer to the file descriptor numbers (0 standard input, 1 standard output, 2 standard error). For example, 2> redirects file descriptor 2, or standard error. &n is the syntax for redirecting to a specific open file. For example 2>&1 redirects 2 (standard error) to 1 (standard output); if 1 has been redirected to a file, 2 goes there too. Other file descriptor numbers are assigned sequentially to other open files, or can be explicitly referenced in the shell scripts. Some of the forms of redirection for the Bourne shell family are:

Note that < and > assume standard input and output, respectively, as the default, so the numbers 0 and 1 can be left off. The form of a command with standard input and output redirection is:

$ command -[options] [arguments] < input file > output file 

Redirection may fail under some circumstances: 1) if you have the variable noclobber set and you attempt to redirect output to an existing file without forcing an overwrite, 2) if you redirect output to a file you don't have write access to, and 3) if you redirect output to a directory.

Examples:

$ who > names

Direct standard output to a file named names

$ (pwd; ls -l) > out

Direct output of both commands to a file named out

$ pwd; ls -l > out

Direct output of ls command only to a file named out

Input redirection can be useful if you have written a program which expects input from the terminal and you want to provide it from a file. In the following example, myprog, which was written to read standard input and write standard output, is redirected to read myin and write myout.

$ myprog < myin > myout 

You can suppress redirected output and/or error by sending it to the null device, /dev/null. The example shows redirection of standard error only:

$ who 2> /dev/null 

To redirect standard error and output to different files (note that grouping is not necessary in Bourne shell):

$ cat myfile > myout 2> myerror