Using Command Symbols

Old-fashioned programs that take all of their input from a command line and then run unaided can be useful in a multitasking system. You can turn them loose to perform complicated processing in the background while you continue to work with other programs in the foreground.

To work better with other programs, many command-line programs follow a set of conventions that control their interaction:

• By default, programs take all of their input as lines of text typed at the keyboard. But input in the same format also can be redirected from a file or any device capable of sending lines of text.
• By default, programs send all of their output to the screen as lines of text. But output in the same format also can be redirected to a file or another line-oriented device, such as a printer.
• Programs set a number called a return value when they terminate, to indicate the results of the program.

When programs are written according to these rules, you can use the symbols in Table below to control a program's input and output, and to connect or chain programs together.

Command Symbols