redirect.md

Unix Command Input/Output redirection

Most "filter" type Unix commands will read from standard input and write to standard output. The default behavior can be modified in the shell when the command is executed.

Output redirection

In the example below, the contents of the unixfile.txt file are first dumped to standard output (screen) and then redirected to another file.

$ cat unixfile.txt
hello
bye

$ cat unixfile.txt > tmpfile

$ cat tmpfile
hello
bye

Input redirection

The input can also be redirected.

$ cat < unixfile.txt
hello
bye

Chaining commands.

Unix is built on the philosophy that each command should be simple and do only one thing but do it well. Piping is used to chain these simple commands to create complex workflows.

For example, to see how many lines in the file input_users.txt contain the word Alice, we would run the following.

$ cat input_users.txt  | grep Alice | wc -l
    2

The above will "pipe" the output of the cat command to the input of the grep command then "pipe" the output of the grep command to the input of the wc command.

See grep and wc for details about the above commands used in the pipe.