Fedora Linux

loop-commands	do
done	df -h
	done

The for..in control structure is great for looping over a range of values. This loop will display the status of the httpd , ftpd , and NetworkManager services:

for SERVICE in httpd ftpd NetworkManager
do
/sbin/service $SERVICE status
done

for...in is even more useful when the list of values is specified as an ambiguous filename. In this script, the loop is repeated once for each file in the directory /etc/ that ends in .conf :

mkdir backup
for FILE in /etc/*.conf
do
echo 'Backing up the file $FILE...'
cp $FILE backup/
done

For the if and while control structures, a control-command determines the action taken. The control-command can be any command on the system; an exit status of zero is considered TRue and any other exit status is considered false .

For example, the grep command exits with a value of zero if a given pattern is found in the file(s) specified or in the standard input. When combined with an if structure, you can cause a program to take a particular action if a pattern is found. For example, this code displays the message 'Helen is logged in!' if the output of who contains the word helen :

if who | grep -q helen
then
echo 'Helen is logged in!'
fi

The exit status of the last command is taken as the exit status of a pipeline, which is grep in this case. The -q argument to grep suppresses the outputotherwise, matching lines are sent to standard output.  

The built-in command test can be used to test conditions; the exit status will be zero if the condition is TRue . The most common conditional expressions are listed in Table 4-19.

Table 4-19. Common bash conditional operators

Operator	Tests whether...	Example using an environment variable
-f file	File exists and is a regular file	-f '$A'
-d file	File exists and is a directory	-d '$B'
-r file	File exists and is readable	-r '$C'
-w file	File exists and is writable	-w '$D'
-x file	File exists and is executable	-x '$E'
value1 == value2	Strings match	'$F' == 'red'
value1 != value2	Strings don't match	'$G' != 'blue'
value1 -eq value2	Integer values are equal	'$H' -eq 2
value1 -ne value2