Fedora Linux

4.12.1.3. Control structures

Like most programming languages, bash features a number of control structures to enable looping and conditional execution. The three most common control structures are listed in Table 4-18 ; there is also a C-style for loop that I'll discuss in the next section.

Table 4-18. Common bash control structures

Structure	Notes	Example
for variable in list	The variable is assigned the first value in list, and loop-commands are executed. The process is then repeated for all of the other values in list.	# Set X to 'hosts', then display the filename and file contents. Repeat for 'services'
do		for X in hosts services
loop-commands		do
done		echo "==== $X"
		cat /etc/$X
		done
if control-command	If the control-command succeeds, the if-commands are executed; otherwise, the else-commands are executed.	# Tell the user if the text 'test' appears in file1
then		if grep -q test file1
if-commands		then
[else		echo "Found it!"
else-commands]		else
fi		echo "Not found."
		fi
while control-command	As long as control-command executes successfully, loop-commands are repeated.	# Display the free disk space every 2 seconds, forever
do		while sleep 2
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	Integer values are unequal	"$J" -ne 10
value1 -gt value2	value1 integer value is greater than value2	"$K" -gt 25
value1 -ge value2	value1 integer value is greater than or equal to value2	"$L" -ge 25
value1 -lt value2	value1 integer value is less than value2	"$M" -lt 75
value1 -le value2	value1 integer value is less than or equal to value2	"$N" -le 75
expression1 -a expression2	expression1 and expression2 are both true	"$P" -gt 36 -a "$P" -lt 71
expression1 -o expression2	expression1 or expression2 (or both) are true	"$P" -lt 12 -o "$P" -eq 50