Книга: The Linux Command Line
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Part IV. Writing Shell Scripts

, we discussed the PATH environment variable and its effect on how the system searches for executable programs. To recap, the system searches a list of directories each time it needs to find an executable program, if no explicit path is specified. This is how the system knows to execute /bin/ls when we type ls at the command line. The /bin directory is one of the directories that the system automatically searches. The list of directories is held within an environment variable named PATH. The PATH variable contains a colon-separated list of directories to be searched. We can view the contents of PATH:

, we looked at a particularly long example of the find command:

[me@linuxbox ˜]$ find playground \ ( -type f -not -perm 0600 -exec chmod 0600 '{}' ';' \)  -or \( -type d -not -perm 0700 -exec chmod 0700 '{}' ';' \)

This command is a little hard to figure out at first glance. In a script, this command might be easier to understand if written this way:

find playground \         \( \                 -type f \                 -not -perm 0600 \                 -exec chmod 0600 '{}' ';' \         \) \         -or \         \( \                 -type d \                 -not -perm 0700 \                  -exec chmod 0700 '{}' ';' \         \)

Through the use of line continuations (backslash-linefeed sequences) and indentation, the logic of this complex command is more clearly described to the reader. This technique works on the command line, too, though it is seldom used as it is very awkward to type and edit. One difference between a script and the command line is that a script may employ tab characters to achieve indentation, whereas the command line cannot because tabs are used to activate completion.

Configuring vim for Script Writing

The vim text editor has many, many configuration settings. Several common options can facilitate script writing.

:syntax on turns on syntax highlighting. With this setting, different elements of shell syntax will be displayed in different colors when viewing a script. This is helpful for identifying certain kinds of programming errors. It looks cool, too. Note that for this feature to work, you must have a complete version of vim installed, and the file you are editing must have a shebang indicating the file is a shell script. If you have difficulty with :syntax on, try :set syntax=sh instead.

:set hlsearch turns on the option to highlight search results. Say we search for the word echo. With this option on, each instance of the word will be highlighted.

:set tabstop=4 sets the number of columns occupied by a tab character. The default is eight columns. Setting the value to 4 (which is a common practice) allows long lines to fit more easily on the screen.

:set autoindent turns on the auto indent feature. This causes vim to indent a new line the same amount as the line just typed. This speeds up typing on many kinds of programming constructs. To stop indentation, type ctrl-D.

These changes can be made permanent by adding these commands (without the leading colon characters) to your ˜/.vimrc file.

In this first chapter about scripting, we have looked at how scripts are written and made to easily execute on our system. We also saw how we can use various formatting techniques to improve the readability (and thus, the maintainability) of our scripts. In future chapters, ease of maintenance will come up again and again as a central principle in good script writing.

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