Datasheet

Task 1.9: Write Basic Scripts

you can install in Linux as a command—even another script. The following list includes a few

commands that are commonly used in scripts:

Normal file-manipulation commands The file-manipulation commands, such as ls, mv, cp,

and rm, are often used in scripts. You can use these commands to help automate repetitive file

maintenance tasks.

grep This command is described earlier, in “Piping Data between Programs” in Task 1.7. It

locates files that contain specific strings.

find Where grep searches for patterns within the contents of files, find does so based on file-

names, ownership, and similar characteristics. This command is described earlier, in Task 1.4.

cut This command extracts text from fields in a file. It’s frequently used to extract variable

information from a file whose contents are highly patterned. To use it, you pass it one or more

options that control what it cuts followed by one or more filenames. For instance, users’ home

directories appear in the sixth colon-delimited field of the /etc/passwd file. You could there-

fore include cut -f 6 -d “:” /etc/passwd in a script to extract this information.

sed This program provides many of the capabilities of a conventional text editor but via

commands that can be typed at a command prompt or entered in a script.

echo Sometimes a script must provide a message to the user; echo is the tool to accomplish

this goal. You can pass various options to echo or just a string to be shown to the user. For

instance, echo “Press the Enter key” causes a script to display the specified string.

mail The mail command can be used to send email from within a script. Pass it the -s subject

parameter to specify a subject line and give it an email address as the last argument. If it’s used at

the command line, you would then type a message and terminate it with a Ctrl+D. If it’s used from

a script, you might omit the message body entirely or pass it an external file as the message using

input redirection. You might want to use this command to send mail to the superuser about the

actions of a startup script or a script that runs on an automated basis.

Many of these commands are extremely complex. You can consult their man

pages for more information.

Even if you have a full grasp of how to use some key external commands, simply executing

commands you might type at a command prompt is of limited utility. Many administrative

tasks require you to modify what you type at a command, or even what commands you enter,

depending on information from other commands. For this reason, scripting languages include

additional features to help you make your scripts useful.

Using Variables

Variables can help you expand the utility of scripts. A variable is a placeholder in a script for

a value that will be determined when the script runs. Variables’ values can be passed as param-

eters to scripts, generated internally to the scripts, or extracted from the scripts’ environments.

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