Fedora Linux

Using Shell Redirection and Piping

The Unix/Linux philosophy revolves around the concept of programs as building blocks—each one intended to do one job and do it well. Redirection lets you connect these commands to files, and piping enables you to plug commands together like a child’s toy.

How Do I Do That?

Each command has three numbered file descriptors that are opened automatically:

standard input (stdin, file descriptor 0)

The normal input to the program

standard output (stdout, file descriptor 1)

The normal output from the program

standard error (stderr, file descriptor 2)

Error messages from the program

By default, these file descriptors are connected to the terminal, if one is available, so standard input comes from the terminal keyboard, and standard output and standard error go to the terminal screen. Programs may open any other connections they need to read or write files, communicate with other local programs, or communicate with programs over the network.

Tip

Connections to the graphical user interface are created by opening a network connection from the program (client) to the X Window server. This is distinct from the three standard file descriptors.

Redirection

To redirect the output of a program to a file, use the greater-than (>) symbol followed by the name of the file:

$ cal 7 2006
     July 2006      
Su Mo Tu We Th Fr Sa
                   1
 2  3  4  5  6  7  8
 9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29
30 31
$ cal 7 2006 >
                  
                     month.txt
                  
$ cat 
                  
                     month.txt
                  
                    July 2006 Su Mo Tu We Th ...