expr
evaluate arguments as an expression
TLDR
Get the length of a specific string
Get the substring of a string with a specific length
Match a specific substring against an anchored pattern
Get the first char position from a specific set in a string
Calculate a specific mathematic expression
Get the first expression if its value is nonzero and not null otherwise get the second one
Get the first expression if both expressions are nonzero and not null otherwise get zero
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.
REPORTING BUGS
GNU coreutils online help:
<https://www.gnu.org/software/coreutils/>
Report any translation bugs to
<https://translationproject.org/team/>
COPYRIGHT
Copyright © 2022 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 <https://www.gnu.org/software/coreutils/expr> or available locally via: info '(coreutils) expr invocation'
AUTHOR
Written by Mike Parker, James Youngman, and Paul Eggert.