Chris Tyler - Fedora Linux

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

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

So if you wanted to print "Too high!" if the value of the variable A was over 50, you would write:

if test "$A" -gt 50

then

echo "Too high!"

fi

The variable expression $A is quoted in case A has a null value ("") or doesn't existin which case, if unquoted, a syntax error would occur because there would be nothing to the left of -gt .

The square brackets ( [] ) are a synonym for test , so the previous code is more commonly written:

if [ "$A" -gt 50 ]

then

echo "Too high!"

fi

You can also use test with the while control structure. This loop monitors the number of users logged in, checking every 15 seconds until the number of users is equal to or greater than 100, when the loop will exit and the following pipeline will send an email to the email alias alert :

while [ "$(who | wc -l)" -lt 100 ]

do

sleep 15

done

echo "Over 100 users are now logged in!"|mail -s "Overload!" alert

bash provides very limited integer arithmetic capabilities. An expression inside double parentheses (( )) is interpreted as a numeric expression; an expression inside double parentheses preceded by a dollar sign $(( )) is interpreted as a numeric expression that also returns a value.

Inside double parentheses, you can read a variable's value without using the dollar sign (use A=B+C instead of A=$B+$C).

Here's an example using a while loop that counts from 1 to 20 using integer arithmetic:

A=0

while [ "$A" -lt 20 ]

do

(( A=A+1 ))

echo $A

done

The C-style increment operators are available, so this code could be rewritten as:

A=0

while [ "$A" -lt 20 ]

do