Unix Power Tools

36.21
Logical operators
Use these to compare two arguments. Depending on the values, the result
written to standard output can be arg1 (or some portion of it), arg2, or0.
The symbols | and & must be escaped.
| Logical OR; if arg1 has a nonzero (and non-null) value, the output is
arg1; otherwise, the output is arg2.
& Logical AND; if both arg1 and arg2 have a nonzero (and non-null)
value, the output is arg1; otherwise, the output is 0.
: Sort of like grep (13.1); arg2 is a regular expression (32.4) to search for in
arg1. If the arg2 pattern is enclosed in \( \), the output is the portion of
arg1 that matches; otherwise, the output is simply the number of characters
that match. Apattern match always applies to the beginning of
the argument (the ^ symbol (32.5) is assumed by default).
Examples
Division happens first; output is 10:
$ expr 5 + 10 / 2
Addition happens first; output is 7 (truncated from 7.5):
$ expr \( 5 + 10 \) / 2
Add 1 to variable i; this is how variables are incremented in Bourne shell scripts:
i=`expr "$i" + 1`
Output 1 (true) if variable a is the string “hello”:
$ expr "$a" = hello
Output 1 (true) if variable b plus 5 equals 10 or more:
$ expr "$b" + 5 \>= 10
In the examples below, variable p is the string “version.100”. This command
returns the number of characters in p:
$ expr "$p" : '.*' Output is 11
Match all characters and print them:
$ expr "$p" : '\(.*\)' Output is “version.100”
Output the number of lowercase letters matched:
$ expr "$p" : '[a-z]*' Output is 7
Match a string of lowercase letters:
$ expr "$p" : '\([a-z]*\)' Output is “version”
Chapter 36: Shell Programming for the Initiated 763
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