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Linux System Administration
book

Linux System Administration

by Tom Adelstein, Bill Lubanovic
March 2007
Intermediate to advanced content levelIntermediate to advanced
304 pages
11h 12m
English
O'Reilly Media, Inc.
Content preview from Linux System Administration
Useful Elements for bash Scripts
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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 ...
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Publisher Resources

ISBN: 9780596009526Supplemental ContentCatalog PageErrata