

expr1  expr2  
Return the evaluation of expr1 if it is neither an empty string nor zero; otherwise, returns the evaluation of expr2 if it is not an empty string; otherwise, returns zero.  
expr1 & expr2  
Return the evaluation of expr1 if neither expression evaluates to an empty string or zero; otherwise, returns zero.  
expr1 {=, >, >=, <, <=, !=} expr2  
Return the results of integer comparison if both arguments are integers; otherwise, returns the results of string comparison using the localespecific collation sequence. The result of each comparison is 1 if the specified relation is true, or 0 if the relation is false.  
expr1 {+, } expr2  
Return the results of addition or subtraction of integervalued arguments.  
expr1 {*, /, %} expr2  
Return the results of multiplication, integer division, or remainder of integervalued arguments.  
expr1: expr2  
The
":"
operator matches
expr1
against
expr2,
which must be a basic regular expression.
The regular expression is anchored
to the beginning of the string with an implicit
"^".
If the match succeeds and the pattern contains at least one regular expression subexpression "\(...\)", the string corresponding to "\1" is returned; otherwise the matching operator returns the number of characters matched. If the match fails and the pattern contains a regular expression subexpression the null string is returned; otherwise 0.  
Parentheses are used for grouping in the usual manner.
The expr utility makes no lexical distinction between arguments which may be operators and arguments which may be operands. An operand which is lexically identical to an operator will be considered a syntax error. See the examples below for a workaround.
The syntax of the expr command in general is historic and inconvenient. New applications are advised to use shell arithmetic rather than expr.
Unless
.Fx 4.x compatibility is enabled, this version of expr adheres to the POSIX Utility Syntax Guidelines, which require that a leading argument beginning with a minus sign be considered an option to the program. The standard
syntax may be used to prevent this interpretation. However, many historic implementations of expr, including the one in previous versions of
.Fx , will not permit this syntax. See the examples below for portable ways to guarantee the correct interpretation. The check_utility_compat(3) function (with a utility argument of "expr") is used to determine whether backwards compatibility mode should be enabled. This feature is intended for use as a transition and debugging aid, when expr is used in complex scripts which cannot easily be recast to avoid the nonportable usage. Enabling backwards compatibility mode also implicitly enables the e option, since this matches the historic behavior of expr in
.Fx . This option makes number parsing less strict and permits leading white space and an optional leading plus sign. In addition, empty operands have an implied value of zero in numeric context. For historical reasons, defining the environment variable EXPR_COMPAT also enables backwards compatibility mode.
EXPR_COMPAT If set, enables backwards compatibility mode.
The expr utility exits with one of the following values:
0 the expression is neither an empty string nor 0. 1 the expression is an empty string or 0. 2 the expression is invalid.
 The following example (in sh(1) syntax) adds one to the variable a:
a=$(expr $a + 1)
 This will fail if the value of a is a negative number. To protect negative values of a from being interpreted as options to the expr command, one might rearrange the expression:
a=$(expr 1 + $a)
 More generally, parenthesize possiblynegative values:
a=$(expr \( $a \) + 1)
 With shell arithmetic, no escaping is required:
a=$((a + 1))
 This example prints the filename portion of a pathname stored in variable a. Since a might represent the path /, it is necessary to prevent it from being interpreted as the division operator. The // characters resolve this ambiguity.
expr "//$a" : ’.*/\(.*\)’
 With modern sh(1) syntax,
"${a##*/}"
expands to the same value.
The following examples output the number of characters in variable a. Again, if a might begin with a hyphen, it is necessary to prevent it from being interpreted as an option to expr, and a might be interpreted as an operator.
 To deal with all of this, a complicated command is required:
expr \( "X$a" : ".*" \)  1
 With modern sh(1) syntax, this can be done much more easily:
${#a}
expands to the required number.
sh(1), test(1), check_utility_compat(3)
The expr utility conforms to p1003.12008, provided that backwards compatibility mode is not enabled.Backwards compatibility mode performs less strict checks of numeric arguments:
 An empty operand string is interpreted as 0.
 Leading white space and/or a plus sign before an otherwise valid positive numeric operand are allowed and will be ignored.
The extended arithmetic range and overflow checks do not conflict with POSIX’s requirement that arithmetic be done using signed longs, since they only make a difference to the result in cases where using signed longs would give undefined behavior.
According to the POSIX standard, the use of string arguments length, substr, index, or match produces undefined results. In this version of expr, these arguments are treated just as their respective string values.
The e flag is an extension.
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