All the shells listed in Table 11.1 have accompanying man pages, along with other documentation under the /usr/share/doc directory. Some of the documentation can be quite lengthy, but it is generally much better to have too much documentation than too little! The bash shell includes more than 100 pages in its manual, and the zsh shell documentation is so extensive that it includes the zshall meta man page (use man zshall to read this overview)!
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.
grepIf 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).
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.