Flow control in shell scripts

[DarkLordoftheMonkeys]

This is my second tutorial on shell scripting. Before, I talked about shell variables. Now I will go into another important topic which is flow control.

There are six types of control structures in bash: if/then, for, case, while, until, and select.

The if/then statement is much like the equivalent statement in other languages, and is used to test if a condition is true:

if [ condition ]
then
	commands;
elif [ condition ]
then
	commands;
else
	commands;
fi

if [ condition ]; then literally means "If condition is true, then do what comes next." elif (the equivalent of else if) means "If not that but this...". Whatever comes after else is executed if none of the other conditions are true. fi (if spelled backwards) terminates the statement. Note that the elif and else sections are optional, but if, then, and fi are required.

The while loop repeats a series of commands for the period of time that a certain statement is true.

while [ condition ]
do
	commands;
done

The until loop is basically the opposite of a while loop. It repeats a series of commands as long as a condition is false.

until [ condition ]
do
	commands;
done

Example:

#!/bin/bash
declare -i int=0;
until [ $int -eq 10 ]
do
	echo $int;
	int++;
done

Outputs:

0
1
2
3
4
5
6
7
8
9

declare -i int initializes an integer variable int. -eq is the Shell notation for "equals" (for strings you use the = sign). ++ is the operator to increase an integer's value by 1.

The for loop is like the while loop, but rearranged. Instead of:

initialization;
while condition
do
	commands;
	increment;
done

You have:

for((initialization; condition; increment))
do
	commands;
done

This will be familiar to programmers with experience in languages like C, C++, and Java. Note, however, that this loop uses double parenthesis. This is to provide extra protection for the expressions inside.

The for loop can also loop through variables in a list, repeating an action for each list item. For instance:

#!/bin/bash
for param in $*
do
	echo $param;
done

If this script is called like this:

$ source forscript "snake" "camel"

It will output:

snake
camel

The case statement is similar to the switch-case statement of C-like languages, but with one major difference. Instead of testing the value of a variable, it tests to see whether a string matches a certain pattern. In Shell, these patterns are known as globs.

case $file in
	*.html )
		echo "HTML file";;
	*.css )
		echo "CSS stylesheet";;
	*.js )
		echo "Javascript script";;
	*
		echo "Another type of file";;
esac

Here, *.html, *.css, etc. are the globs. The * means any string of characters, so *.html would match either browser-stats.html or OS-stats.html.

The last flow control statement is the select statement, which creates a menu of items in a list.

#!/bin/bash
select file in *
do
	echo $file;
	break;
done

This script will create a menu of files in the current directory. When the user selects one of the menu items, the name of the file will be output. break is necessary because it prevents the statement from looping indefinitely.

[WingedPanther]

Nice +rep