expr : command line Unix utility

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expr
(인용)expr is a command line Unix utility which evaluates an expression and outputs the corresponding value.
Syntax: expr (expression)

좋은 튜토리얼 :
Advanced Bash-Scripting Guide: Manipulating Strings
Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command.

http://www.softpanorama.org/Utilities/expr.shtml
The external expr command evaluates and compares expressions. An expression is composed of operands and operators...

Substrings
(인용) Often times a programmer needs to be able to get a substring from a variable at a given position. In unix you can use the expr command to do this with the substr parameter...

매뉴얼 :
expr: Evaluate expressions