Книга: Fedora™ Unleashed, 2008 edition

The Shell Command Line

The Shell Command Line

Having a basic understanding of the capabilities of the shell command line can help you write better shell scripts. If, after you have finished reading this short introduction, you want to learn more about the command line, check out Chapter 32, "Command-Line Master Class." You can use the shell command line to perform a number of different tasks, including

? Getting data from and sending data to a file or command, known as input and output redirection.

? Feeding or filtering a program's output to another command (called using pipes).

A shell can also have built-in job-control commands to launch the command line as a background process, suspend a running program, selectively retrieve or kill running or suspended programs, and perform other types of process control.

Multiple commands can be run on a single command line, with a semicolon to separate commands:

$ w ; free ; df
6:02pm up 4 days, 24 min, 1 user, load average: 0.00, 0.00, 0.00
USER  TTY   FROM             LOGIN@ IDLE  JCPU  PCPU  WHAT
bball pts/0 shuttle.home.org 1:14pm 0.00s 0.57s 0.01s w
       total         used    free shared buffers cached
Mem:  190684       184420    6264     76   17620 142820
-/+ buffers/cache:  23980  166704
Swap: 1277156        2516 1274640
Filesystem                  1k-blocks             Used Available Use% Mounted on
/dev/hda1 11788296 4478228 6711248 41% /
none                            95340                   0 95340 0% /dev/shm

This example displays the output of the w, free, and df commands. You can extend long shell command lines inside shell scripts or at the command line if you use the backslash character (). For example,

$ echo ""this is a long
> command line and"" ; echo ""shows that multiple commands
> may be strung out.""
this is a long command line and
shows that multiple commands may be strung out.

The first three lines of this example are a single command line. In that single line are two instances of the echo command. Note that when you use the backslash as a line-continuation character, it must be the last character on the command line (or in your shell script, as you will see later in this chapter).

Using the basic features of the shell command line is easy, but mastering use of all features can be difficult. Entire books have been devoted to using shells, writing shell scripts, and using pattern-matching expressions. The following sections provide an overview of some features of the shell command line relating to writing scripts.

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