# FreeBSD Manual Pages

EXPR(1) BSD General Commands Manual EXPR(1)NAMEexpr-- evaluate expressionSYNOPSISexpr[-e]expressionDESCRIPTIONTheexprutility evaluatesexpressionand writes the result on standard output. All operators and operands must be passed as separate arguments. Several of the operators have special meaning to command interpreters and must therefore be quoted appropriately. All integer operands are interpreted in base 10 and must consist of only an optional leading minus sign fol- lowed by one or more digits (unless less strict parsing has been enabled for backwards compatibility with prior versions ofexprin FreeBSD). Arithmetic operations are performed using signed integer math with a range according to the Cintmax_tdata type (the largest signed integral type available). All conversions and operations are checked for over- flow. Overflow results in program termination with an error message on stdout and with an error status. The-eoption enables backwards compatible behaviour as detailed below. Operators are listed below in order of increasing precedence; all are left-associative. Operators with equal precedence are grouped within symbols `{' and `}'.expr1|expr2Return the evaluation ofexpr1if it is neither an empty string nor zero; otherwise, returns the evaluation ofexpr2if it is not an empty string; otherwise, returns zero.expr1&expr2Return the evaluation ofexpr1if neither expression evaluates to an empty string or zero; otherwise, returns zero.expr1{=, >, >=, <, <=, !=}expr2Return the results of integer comparison if both arguments are integers; otherwise, returns the results of string comparison us- ing the locale-specific collation sequence. The result of each comparison is 1 if the specified relation is true, or 0 if the relation is false.expr1{+, -}expr2Return the results of addition or subtraction of integer-valued arguments.expr1{*, /, %}expr2Return the results of multiplication, integer division, or re- mainder of integer-valued arguments.expr1:expr2The ":" operator matchesexpr1againstexpr2, which must be a ba- sic 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 regu- lar 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. Theexprutility 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 work-around. The syntax of theexprcommand in general is historic and inconvenient. New applications are advised to use shell arithmetic rather thanexpr.CompatibilitywithpreviousimplementationsUnless FreeBSD 4.x compatibility is enabled, this version ofexpradheres to the POSIX Utility Syntax Guidelines, which require that a leading ar- gument beginning with a minus sign be considered an option to the pro- gram. The standard--syntax may be used to prevent this interpretation. However, many historic implementations ofexpr, including the one in pre- vious versions of FreeBSD, will not permit this syntax. See the examples below for portable ways to guarantee the correct interpretation. The check_utility_compat(3) function (with autilityargument 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, whenexpris used in complex scripts which cannot easily be recast to avoid the non-portable usage. Enabling backwards compatibility mode also im- plicitly enables the-eoption, since this matches the historic behavior ofexprin FreeBSD. This option makes number parsing less strict and permits leading white space and an optional leading plus sign. In addi- tion, 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.ENVIRONMENTEXPR_COMPAT If set, enables backwards compatibility mode.EXIT STATUSTheexprutility 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.EXAMPLES+oThe following example (in sh(1) syntax) adds one to the variablea: a=$(expr $a + 1)+oThis will fail if the value ofais a negative number. To protect negative values ofafrom being interpreted as options to theexprcommand, one might rearrange the expression: a=$(expr 1 + $a)+oMore generally, parenthesize possibly-negative values: a=$(expr \( $a \) + 1)+oWith shell arithmetic, no escaping is required: a=$((a + 1))+oThis example prints the filename portion of a pathname stored in variablea. Sinceamight represent the path/, it is necessary to prevent it from being interpreted as the division operator. The // characters resolve this ambiguity. expr "//$a" : '.*/\(.*\)'+oWith modern sh(1) syntax, "${a##*/}" expands to the same value. The following examples output the number of characters in variablea. Again, ifamight begin with a hyphen, it is necessary to prevent it from being interpreted as an option toexpr, andamight be interpreted as an operator.+oTo deal with all of this, a complicated command is required: expr \( "X$a" : ".*" \) - 1+oWith modern sh(1) syntax, this can be done much more easily: ${#a} expands to the required number.SEE ALSOsh(1), test(1), check_utility_compat(3)STANDARDSTheexprutility conforms to IEEE Std 1003.1-2008 ("POSIX.1"), provided that backwards compatibility mode is not enabled. Backwards compatibility mode performs less strict checks of numeric argu- ments:+oAn empty operand string is interpreted as 0.+oLeading white space and/or a plus sign before an otherwise valid pos- itive 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 argumentslength,substr,index, ormatchproduces undefined results. In this version ofexpr, these arguments are treated just as their respective string values. The-eflag is an extension. BSD February 25, 2012 BSD

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