Expr

expr is a command line utility which evaluates an expression and outputs the corresponding value.

expr evaluates integer or string expressions, including pattern matching regular expressions. Most of the challenge posed in writing expressions is preventing the command line shell from acting on characters intended for expr to process.

The operators available:

• for integers: addition, subtraction, multiplication, division and modulus
• for strings: find substring, find regular expression, find a set of characters in a string, length of string
• for either: comparison (equal, not equal, less than, etc.)

Also, boolean expressions involving and and or, such as

```expr length "abcdef" \< 5   "|"   15 - 4 ">" 8
```

outputs "1". This is because length "abcdef" is 6, which is not less than (the backslash escapes a less than from the shell) 5 (so the left side of the | returns zero). But 15 minus 4 is 11 and is greater than 8, so the right side is true, which makes the or true, so 1 is the result. (The return value—the program exit status—is zero for this example.)

man page

```Usage: expr EXPRESSION
or: expr OPTION
```

Print the value of EXPRESSION to standard output. A blank line below separates increasing precedence groups. EXPRESSION may be:

```  ARG1 | ARG2       ARG1 if it is neither null nor 0, otherwise ARG2
ARG1 & ARG2       ARG1 if neither argument is null or 0, otherwise 0
ARG1 < ARG2       ARG1 is less than ARG2
ARG1 <= ARG2      ARG1 is less than or equal to ARG2
ARG1 = ARG2       ARG1 is equal to ARG2
ARG1 != ARG2      ARG1 is unequal to ARG2
ARG1 >= ARG2      ARG1 is greater than or equal to ARG2
ARG1 > ARG2       ARG1 is greater than ARG2
ARG1 + ARG2       arithmetic sum of ARG1 and ARG2
ARG1 - ARG2       arithmetic difference of ARG1 and ARG2
ARG1 * ARG2       arithmetic product of ARG1 and ARG2
ARG1 / ARG2       arithmetic quotient of ARG1 divided by ARG2
ARG1 % ARG2       arithmetic remainder of ARG1 divided by ARG2

STRING : REGEXP   anchored pattern match of REGEXP in STRING

match STRING REGEXP        same as STRING : REGEXP
substr STRING POS LENGTH   substring of STRING, POS counted from 1
index STRING CHARS         index in STRING where any CHARS is found, or 0
length STRING              length of STRING
+ TOKEN                    interpret TOKEN as a string, even if it is a
keyword like `match' or an operator like `/'

( EXPRESSION )             value of EXPRESSION
```

Beware that many operators need to be escaped or quoted for shells. Comparisons are arithmetic if both ARGs are numbers, else lexicographical. Pattern matches return the string matched between \( and \) or null; if \( and \) are not used, they return the number of characters matched or 0.