expr command in UNIX

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expr EXPRESSION
expr OPTION

Introduction

expr command evaluates arithmetical expressions taking string arguments.

Options

expr command print the value of EXPRESSION to standard output.

EXPRESSION may be:

  • ARG1 | ARG2: ARG1 if it is neither null nor 0, otherwise ARG2
  • ARG1 & ARG2: ARG1 if neither argument is null nor 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
  • 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 /

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.

Exit status is:

  • 0 if EXPRESSION is neither null nor 0
  • 1 if EXPRESSION is null or 0
  • 2 if EXPRESSION is syntactically invalid
  • 3 if an error occurred

Examples

Logical

expr 3 '|' 2
3 # ARG1 is not null nor 0

expr 0 '|' 2
2 # ARG1 is 0

expr 0 '&' 2
0 # ARG1 is 0

expr 4 '&' 2
4 # ARG1 and ARG2 are not null nor 0

expr 4 '&' 0
0 # ARG2 is 0

 expr 4 '&' ''
0 # ARG2 is null

Comparison

# like '==' in most programming languages
expr 55 = 23
0 # return 0: given numbers are not equal

expr 55 = 55
1 # return 1: given numbers are equal

expr 3 '<' 2
0 # return 0: left-side number is not minor than right-side number

expr 3 '>' 2
1 # return 1: left-side number is major than right-side number

Arithmetical

expr 55 + 96 - 23
128

a=82; b="$a + 6"; echo $b
82 + 6

expr $b
88

expr $a - $b
6

String

# evaluate the length of a string
expr length "Hello, World!"
13

# extract substrings from string variables
hello="HelloWorld"
expr substr $hello 3 4
lloW

# in sh and bash index starts at 1
expr index $hello W
6

Quotes

Manual reference: