expr man page
expr — evaluate expressions
TL;DR
 Get string length:
expr length string
 Evaluate logical or math expression with an operator ('+', '', '*', '&', '', etc.). Special symbols should be escaped:
expr first_argument operator second_argument
 Get position of the first character in 'string' that matches 'substring':
echo $(expr index string substring)
 Extract part of the string:
echo $(expr substr string position_to_start number_of_characters
 Extract part of the string which matches a regular expression:
echo $(expr string : '\(regular_expression\)')
Synopsis
expr EXPRESSION
expr OPTION
Description
 help
display this help and exit
 version
output version information and exit
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.
Exit status is 0 if EXPRESSION is neither null nor 0, 1 if EXPRESSION is null or 0, 2 if EXPRESSION is syntactically invalid, and 3 if an error occurred.
Author
Written by Mike Parker, James Youngman, and Paul Eggert.
Reporting Bugs
GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Report expr translation bugs to <https://translationproject.org/team/>
Copyright
Copyright © 2017 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
See Also
Full documentation at: <https://www.gnu.org/software/coreutils/expr>
or available locally via: info '(coreutils) expr invocation'