input/output redirectioncomputing dictionary

<operating system> In Unix, to send output from a process to different file or device or to another process via a pipe, or to have a process read its input from a different file, device or pipe. Some other operating systems have similar facilities.

To redirect input to come from a file instead of the keyboard, use "<":

myprog < myfile

Similarly to redirect output to a file instead of the screen:

ls > filelist

A pipe redirects the output of one process directly into the input of another

who | wc -l

A common misuse by beginners is

cat myfile | myprog

Which is more or less equivalent to "myprog < myfile" except that it introduces an extra unnecessary cat process and buffer space for the pipe. Even the "<" is unnecessary with many standard Unix commands since they accept input file names as command line arguments anyway.

Unix's concept of standard input/output and I/O redirection make it easy to combine simple processes in powerful ways and to use the same commands for different purposes.

(01 Apr 1998)

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