Datasheet

Task 1.9: Write Basic Scripts

For instance, consider Listing 1.4, which displays the current IP address of the computer on

which it runs. This script uses the variable $ip, which is extracted from the output of ifconfig

using grep and cut commands. (The trailing backslash on the second line of the script indicates

that the following line is a continuation of the preceding line.) When assigning a value to a vari-

able from the output of a command, that command should be enclosed in back-quote characters

(`), which appear on the same key as the tilde (~) on most keyboards. These are not ordinary sin-

gle quotes, which appear on the same key as the regular quote character (") on most keyboards.

Listing 1.4: Script Demonstrating Assignment and Use of Variables

#!/bin/sh

ip=`ifconfig eth0 | grep inet | cut -f 2 -d ":" | \

cut -f 1 -d " "`

echo "Your IP address is $ip"

Listing 1.4 relies on the networking command ifconfig, which is described in more detail

in Phase 6. You can type ifconfig by itself to see what its output includes. The second line

of Listing 1.4 uses grep and cut to isolate the IP address from the rest of the ifconfig output.

Scripts like Listing 1.4, which obtain information from running one or more commands,

are useful in configuring features that rely on system-specific information or information that

varies with time. You might use a similar approach to obtain the current hostname (using the

hostname command), the current time (using date), the total time the computer’s been run-

ning (using uptime), free disk space (using df), and so on. When combined with conditional

expressions (described shortly), variables become even more powerful because then your

script can perform one action when one condition is met and another in some other case. For

instance, a script that installs software could check free disk space and abort the installation

if there’s not enough disk space available.

One special type of variable was mentioned earlier in this chapter: environment variables.

Environment variables are assigned and accessed just like shell script variables. The difference

is that the script or command that sets an environment variable uses the export command to

make the value of the variable accessible to programs launched from the shell or shell script

that made the assignment. Environment variables are most often set in shell startup scripts, but

the scripts you use can access them. For instance, if your script calls X programs, it might

check for the presence of a valid $DISPLAY environment variable and abort if it finds that this

variable isn’t set. By convention, environment variable names are all uppercase, whereas non-

environment shell script variables are all lowercase or mixed case.

Using Conditional Expressions

Scripting languages support several types of conditional expressions. These enable a script to

perform one of several actions contingent on some condition—typically the value of a variable.

One common command that uses conditional expressions is if, which allows the system to take

one of two actions depending on whether some condition is true. The if keyword’s conditional

expression appears in brackets after the if keyword and can take many forms. For instance,

-f file is true if file exists and is a regular file; -s file is true if file exists and has a size

greater than 0; and string1 = string2 is true if the two strings have the same values.

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