The Linux Command Line (2012)

Part IV. Writing Shell Scripts

Chapter 31. Flow Control: Branching with case

In this chapter, we will continue to look at flow control. In Chapter 28, we constructed some simple menus and built the logic used to act on a user’s selection. To do this, we used a series of if commands to identify which of the possible choices had been selected. This type of construct appears frequently in programs, so much so that many programming languages (including the shell) provide a flow-control mechanism for multiple-choice decisions.

case

The bash multiple-choice compound command is called case. It has the following syntax:

case word in
[pattern [| pattern]...) commands ;;]...
esac

If we look at the read-menu program from Chapter 28, we see the logic used to act on a user’s selection

#!/bin/bash

# read-menu: a menu driven system information program

clear
echo "
Please Select:

1. Display System Information
2. Display Disk Space
3. Display Home Space Utilization
0. Quit
"
read -p "Enter selection [0-3] > "

if [[ $REPLY =˜ ^[0-3]$ ]]; then
if [[ $REPLY == 0 ]]; then
echo "Program terminated."
exit
fi
if [[ $REPLY == 1 ]]; then
echo "Hostname: $HOSTNAME"
uptime
exit
fi
if [[ $REPLY == 2 ]]; then
df -h
exit
fi
if [[ $REPLY == 3 ]]; then
if [[ $(id -u) -eq 0 ]]; then
echo "Home Space Utilization (All Users)"
du -sh /home/*
else
echo "Home Space Utilization ($USER)"
du -sh $HOME
fi
exit
fi
else
echo "Invalid entry." >&2
exit 1
fi

Using case, we can replace this logic with something simpler:

#!/bin/bash

# case-menu: a menu driven system information program

clear
echo "
Please Select:

1. Display System Information
2. Display Disk Space
3. Display Home Space Utilization
0. Quit
"
read -p "Enter selection [0-3] > "

case $REPLY in
0) echo "Program terminated."
exit
;;
1) echo "Hostname: $HOSTNAME"
uptime
;;
2) df -h
;;
3) if [[ $(id -u) -eq 0 ]]; then
echo "Home Space Utilization (All Users)"
du -sh /home/*
else
echo "Home Space Utilization ($USER)"
du -sh $HOME
fi
;;
*) echo "Invalid entry" >&2
exit 1
;;
esac

The case command looks at the value of word—in our example, the value of the REPLY variable—and then attempts to match it against one of the specified patterns. When a match is found, the commands associated with the specified pattern are executed. After a match is found, no further matches are attempted.

Patterns

The patterns used by case are the same as those used by pathname expansion. Patterns are terminated with a ) character. Table 31-1 shows some valid patterns.

Table 31-1. case Pattern Examples

Here is an example of patterns at work:

#!/bin/bash

read -p "enter word > "

case $REPLY in
[[:alpha:]]) echo "is a single alphabetic character." ;;
[ABC][0-9]) echo "is A, B, or C followed by a digit." ;;
???) echo "is three characters long." ;;
*.txt) echo "is a word ending in '.txt'" ;;
*) echo "is something else." ;;
esac

Combining Multiple Patterns

It is also possible to combine multiple patterns using the vertical pipe character as a separator. This creates an “or” conditional pattern. This is useful for such things as handling both upper- and lowercase characters. For example:

#!/bin/bash

# case-menu: a menu driven system information program

clear
echo "
Please Select:

A. Display System Information
B. Display Disk Space
C. Display Home Space Utilization
Q. Quit
"
read -p "Enter selection [A, B, C or Q] > "

case $REPLY in
q|Q) echo "Program terminated."
exit
;;
a|A) echo "Hostname: $HOSTNAME"
uptime
;;
b|B) df -h
;;
c|C) if [[ $(id -u) -eq 0 ]]; then
echo "Home Space Utilization (All Users)"
du -sh /home/*
else
echo "Home Space Utilization ($USER)"
du -sh $HOME
fi
;;
*) echo "Invalid entry" >&2
exit 1
;;
esac

Here, we modify the case-menu program to use letters instead of digits for menu selection. Notice that the new patterns allow for entry of both upper- and lowercase letters.

Final Note

The case command is a handy addition to our bag of programming tricks. As we will see in the next chapter, it’s types of problems.