Linux System Administration

Useful Elements for bash Scripts

|

219

Arithmetic

bash is heavily weighted toward text such as commands, arguments, and filenames.

It can evaluate the usual arithmetic expressions (using

+, -, *, /, and other operators)

by surrounding them with a pair of double parentheses:

((expression)). Because

many arithmetic characters—including

*, (, and )—are specially interpreted by the

shell, it’s best to quote shell arguments if they will be treated as math expressions in

the script:

admin@server1:~$ cat arith

#!/bin/bash

answer=$(( $* ))

echo $answer

admin@server1:~$ ./arith "(8+1)*(7-1)-60"

-6

admin@server1:~$ ./arith "2**60"

1152921504606846976

admin@server1:~$

The latest version of bash supports 64-bit integers (–9223372036854775808 to

9223372036854775807). Older versions support only 32-bit integers (with a puny

range of –2147483648 to 2147483647). Floating-point numbers are not supported.

Scripts that need floating-point or more advanced operators can use an external pro-

gram such as bc.

In arithmetic expressions, you can use variables without the

$ character that would

be used to substitute their values in other settings:

admin@server1:~$ cat arithexp

#!/bin/bash

a=$1

b=$(( a+2 ))

echo "$a + 2 = $b"

c=$(( a*2 ))

echo "$a * 2 = $c"

admin@server1:~$ ./arithexp 6

6 + 2 = 8

6 * 2 = 12

admin@server1:~$

If...

Given expressions, you can execute different chunks of code depending on the

results of tests. bash uses the

if ...