Fedora™ Unleashed, 2008 edition

/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.

Grokking grep

If you plan to develop shell scripts to expand the capabilities of pattern-matching commands such as grep, you will benefit from learning more about using expressions. One of the definitive guides to using the pattern-matching capabilities of Unix and Linux commands is Mastering Regular Expressions by Jeffrey E. F. Freidl (O'Reilly), ISBN: 0-596-00289-0.

Shell Pattern-Matching Support

The shell command line allows you to use strings of specially constructed character patterns for wildcard matches. This is a simpler capability than that supported by GNU utilities such as grep, which can use more complex patterns, known as expressions, to search through files or directories or to filter data input to or out of commands.

The shell's pattern strings can be simple or complex, but even using a small subset of the available characters in simple wildcards can yield constructive results at the command line. Some common characters used for shell pattern matching are the following:

> * —Matches any character. For example, to find all files in the current directory ending in .txt, you could use

$ ls *.txt

> ? — Matches a single character. For example, to find all files in the current directory ending in the extension .d?c (where ? could be 0-9, a-z, or A-Z),

$ ls *.d?c

> [xxx] or [x-x] — Matches a range of characters. For example, to list all files in a directory with names containing numbers,

$ ls *[0-9]*

> x — Matches or escapes a character such as ? or a tab character. For example, to create a file with a name containing a question mark,

$ touch foo?

Note that the shell might not interpret some characters or regular expressions in the same manner as a Linux command, and mixing wildcards and regular expressions in shell scripts can lead to problems unless you're careful. For example, finding patterns in text is best left to regular expressions used with commands such as grep; simple wildcards should be used for filtering or matching filenames on the command line. And although both Linux command expressions and shell scripts can recognize the backslash as an escape character in patterns, the dollar sign ($) has two wildly different meanings (single-character pattern matching in expressions and variable assignment in scripts).

CAUTION

Make sure that you read your command carefully when using wildcards; an all-too-common error is to type something like rm -rf * .txt with a space between the * and the .txt. By the time you wonder why the command is taking so long, bash will already have deleted most of your files. The problem is that it treats the * and the .txt separately. * matches everything, so bash deletes all your files.

Piping Data

Many Linux commands can be used in concert in a single, connected command line to transform data from one form to another. Stringing Linux commands together in this fashion is known as using or creating pipes. Pipes are created on the command line with the bar operator (|). For example, a pipe can be used to perform a complex task from a single command line like this:

$ find /d2 -name '*.txt' -print | xargs cat |
tr ' ' '
' | sort | uniq >output.txt

This example takes the output of the find command to feed the cat command (via xargs) the name of all text files under the /d2 command. The content of all matching files is then fed through the tr command to change each space in the data stream into a carriage return. The stream of words is then sorted, and identical adjacent lines are removed with the uniq command. The output, a raw list of words, is then saved in the file named output.txt.

Background Processing

The shell enables you to start a command and then launch it into the background as a process by using an ampersand (&) at the end of a command line. This technique is often used at the command line of an X terminal window to start a client and return to the command line. For example, to launch another terminal window using the xterm client,

$ xterm &
[3] 1437

The numbers echoed back show a number (3 in this example), which is a job number, or reference number for a shell process, and a Process ID number, or PID (1437 in this example). You can kill the xterm window session by using the shell's built-in kill command, along with the job number like this:

$ kill %3

Or the process can be killed with the kill command, along with the PID, like so:

$ kill 1437

Background processing can be used in shell scripts to start commands that take a long time, such as