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36.21. Quick Reference: expr
expr is a very handy tool in shell programming, since it provides the ability to evaluate a wide range of arithmetic, logical, and relational expressions. It evaluates its arguments as expressions and prints the result.
36.21.1. Syntax
Here's the syntax. The [brackets] mean "optional"; don't type the brackets:
expr arg1 operator arg2 [ operator arg3 ... ]
Arguments and operators must be separated by spaces. In many cases, an argument is an integer, typed literally or represented by a shell variable. There are three types of operators: arithmetic, relational, and logical.
Exit status (Section 35.12) values for expr are 0 if the expression evaluates nonzero and non-null, 1 if the expression evaluates to 0 or null, and 2 if the expression is invalid.
- Arithmetic operators
-
Use these to produce mathematical expressions whose results are
printed:
- +
- Add arg2 to arg1.
- -
- Subtract arg2 from arg1.
- *
- Multiply the arguments.
- /
- Divide arg1 by arg2.
- %
- Take the remainder when arg1 is divided by arg2 (modulus).
Addition and subtraction are evaluated last, unless they are grouped inside parentheses. The symbols *, (, and ) have meaning to the shell, so they must be escaped (preceded by a backslash or enclosed in quotes).
- Relational operators
- Use these to compare two arguments. Arguments can also be words, in which case comparisons assume a < z and A < Z. If the comparison statement is true, expr writes 1 to standard output (Section 43.1); if false, it writes 0. The symbols > and < must be escaped.
- 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, or 0. 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 (Section 13.1); arg2 is a regular expression (Section 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. A pattern match always applies to the beginning of the argument (the ^ symbol (Section 32.5) is assumed by default).
36.21.2. 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"
Truncate $x if it contains five or more characters; if not, just output $x. (Logical OR uses the second argument when the first one is 0 or null, i.e., when the match fails.)
$ expr "$x" : '\(.....\)' "$x"
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| 36.20. Turn Off echo for "Secret" Answers |  | 36.22. Testing Characters in a String with expr |
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