expr - evaluate arguments as an expression
The expr utility will evaluate an expression and write the result to standard output.
The single expression evaluated by expr will be formed from the operands, as described in the EXTENDED DESCRIPTION section. Each of the expression operator symbols:and the symbols integer and string in the table must be provided as separate arguments to expr.
( ) | & = > >= < <= != + - * / % :
The following environment variables affect the execution of expr:
- Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.
- If set to a non-empty string value, override the values of all the other internationalisation variables.
- Determine the locale for the behaviour of ranges, equivalence classes and multi-character collating elements within regular expressions and by the string comparison operators.
- Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- as opposed to multi-byte characters in arguments) and the behaviour of character classes within regular expressions.
- Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
- Determine the location of message catalogues for the processing of LC_MESSAGES .
The expr utility will evaluate the expression and write the result to standard output. The character 0 will be written to indicate a zero value and nothing will be written to indicate a null string.
Used only for diagnostic messages.
The formation of the expression to be evaluated is shown in the following table. The symbols expr, expr1 and expr2 represent expressions formed from integer and string symbols and the expression operator symbols (all separate arguments) by recursive application of the constructs described in the table. The expressions are listed in order of increasing precedence, with equal-precedence operators grouped between horizontal lines. All of the operators are left-associative.
Expression Description expr1 | expr2 Returns the evaluation of if it is neither null nor zero; otherwise, returns the evaluation of expr1 & expr2 Returns the evaluation of if neither expression evaluates to null or zero; otherwise, returns zero. Returns the result of a decimal integer comparison if both arguments are integers; otherwise, returns the result of a string comparison using the locale-specific collation sequence. The result of each comparison will be 1 if the specified relationship is true, or 0 if the relationship is false. expr1 = expr2 Equal. expr1 > expr2 Greater than. expr1 >= expr2 Greater than or equal. expr1 < expr2 Less than. expr1 <= expr2 Less than or equal. expr1 != expr2 Not equal. expr1 + expr2 Addition of decimal integer-valued arguments. expr1 - expr2 Subtraction of decimal integer-valued arguments. expr1 * expr2 Multiplication of decimal integer-valued arguments. expr1 / expr2 Integer division of decimal integer-valued arguments, producing an integer result. expr1 % expr2 Remainder of integer division of decimal integer-valued arguments. expr1 : expr2 Matching expression. See below. ( expr ) Grouping symbols. Any expression can be placed within parentheses. Parentheses can be nested to a depth of integer An argument consisting only of an (optional) unary minus followed by digits. string A string argument. See below.
Matching ExpressionThe ":" matching operator will compare the string resulting from the evaluation of expr1 with the regular expression pattern resulting from the evaluation of expr2. Regular expression syntax is that defined in the XBD specification, Basic Regular Expressions , except that all patterns are anchored to the beginning of the string (that is, only sequences starting at the first character of a string will be matched by the regular expression) and, therefore, it is unspecified whether "^" is a special character in that context. Usually, the matching operator will return a string representing the number of characters matched (0 on failure). Alternatively, if the pattern contains at least one regular expression subexpression [\(...\)], the string corresponding to \1 will be returned.
String OperandA string argument is an argument that cannot be identified as an integer argument or as one of the expression operator symbols shown in the OPERANDS section.
The use of string arguments length, substr, index or match produces unspecified results.
The following exit values are returned:
- The expression evaluates to neither null nor zero.
- The expression evaluates to null or zero.
- Invalid expression.
- An error occurred.
After argument processing by the shell, expr is not required to be able to tell the difference between an operator and an operand except by the value. If $a is "=", the command:looks like:
expr $a = '='as the arguments are passed to expr (and they all may be taken as the "=" operator). The following works reliably:
expr = = =
expr X$a = X=
Also note that this specification permits implementations to extend utilities. The expr utility permits the integer arguments to be preceded with a unary minus. This means that an integer argument could look like an option. Therefore, the portable application must employ the -- construct of Guideline 10 of the XBD specification, Utility Syntax Guidelines to protect its operands if there is any chance the first operand might be a negative integer (or any string with a leading minus).
The expr utility has a rather difficult syntax:
- Many of the operators are also shell control operators or reserved words, so they have to be escaped on the command line.
- Each part of the expression is composed of separate arguments, so liberal usage of blank characters is required. For example:
Invalid Valid expr 1+2 expr 1 + 2 expr "1 + 2" expr 1 + 2 expr 1 + (2 * 3) expr 1 + \( 2 \* 3 \)
In many cases, the arithmetic and string features provided as part of the shell command language are easier to use than their equivalents in expr. Newly written scripts should avoid expr in favour of the new features within the shell. See
Parameters and Variablesand Arithmetic Expansion.
The following command:adds 1 to the variable a.
a=$(expr $a + 1)
The following command, for $a equal to either /usr/abc/file or just file:returns the last segment of a pathname (that is, file). Applications should avoid the character "/" used alone as an argument: expr may interpret it as the division operator.
expr $a : '.*/\(.*\)' \| $a
The following command:is a better representation of the previous example. The addition of the // characters eliminates any ambiguity about the division operator and simplifies the whole expression. Also note that pathnames may contain characters contained in the IFS variable and should be quoted to avoid having $a expand into multiple arguments.
expr "//$a" : '.*/\(.*\)'
The following command:returns the number of characters in VAR.
expr "$VAR" : '.*'
The IEEE PASC 1003.2 Interpretations Committee has forwarded concerns about parts of this interface definition to the IEEE PASC Shell and Utilities Working Group which is identifying the corrections. A future revision of this specification will align with IEEE Std. 1003.2b when finalised.