1EXPR(P) POSIX Programmer's Manual EXPR(P)
2
6 expr - evaluate arguments as an expression
7
9 expr operand
10
12 The expr utility shall evaluate an expression and write the result to
13 standard output.
14
16 None.
17
19 The single expression evaluated by expr shall be formed from the oper‐
20 ands, as described in the EXTENDED DESCRIPTION section. The application
21 shall ensure that each of the expression operator symbols:
22 ( ) | & = > >= < <= != + - * / % :
25 and the symbols integer and string in the table are provided as sepa‐
27 rate arguments to expr.
28
30 Not used.
31
33 None.
34
36 The following environment variables shall affect the execution of expr:
37 LANG Provide a default value for the internationalization variables
39 that are unset or null. (See the Base Definitions volume of
40 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
41 ables for the precedence of internationalization variables used
42 to determine the values of locale categories.)
43 LC_ALL If set to a non-empty string value, override the values of all
45 the other internationalization variables.
46 LC_COLLATE
48 Determine the locale for the behavior of ranges, equivalence
50 classes, and multi-character collating elements within regular
51 expressions and by the string comparison operators.
52 LC_CTYPE
54 Determine the locale for the interpretation of sequences of
55 bytes of text data as characters (for example, single-byte as
56 opposed to multi-byte characters in arguments) and the behavior
57 of character classes within regular expressions.
58 LC_MESSAGES
60 Determine the locale that should be used to affect the format
61 and contents of diagnostic messages written to standard error.
62 NLSPATH
64 Determine the location of message catalogs for the processing of
65 LC_MESSAGES .
66
69 Default.
70
72 The expr utility shall evaluate the expression and write the result,
73 followed by a <newline>, to standard output.
74
76 The standard error shall be used only for diagnostic messages.
77
79 None.
80
82 The formation of the expression to be evaluated is shown in the follow‐
83 ing table. The symbols expr, expr1, and expr2 represent expressions
84 formed from integer and string symbols and the expression operator sym‐
85 bols (all separate arguments) by recursive application of the con‐
86 structs described in the table. The expressions are listed in order of
87 increasing precedence, with equal-precedence operators grouped between
88 horizontal lines. All of the operators shall be left-associative.
89 Expression Description
91 expr1 | expr2 Returns the evaluation of expr1 if it is
92 neither null nor zero; otherwise,
93 returns the evaluation of expr2 if it is
94 not null; otherwise, zero.
95 expr1 & expr2 Returns the evaluation of expr1 if nei‐
96 ther expression evaluates to null or
97 zero; otherwise, returns zero.
98 Returns the result of a decimal integer
99 comparison if both arguments are inte‐
100 gers; otherwise, returns the result of a
101 string comparison using the locale-spe‐
102 cific collation sequence. The result of
103 each comparison is 1 if the specified
104 relationship is true, or 0 if the rela‐
105 tionship is false.
106 expr1 = expr2 Equal.
107 expr1 > expr2 Greater than.
108 expr1 >= expr2 Greater than or equal.
109 expr1 < expr2 Less than.
110 expr1 <= expr2 Less than or equal.
111 expr1 != expr2 Not equal.
112 expr1 + expr2 Addition of decimal integer-valued argu‐
113 ments.
114 expr1 - expr2 Subtraction of decimal integer-valued
115 arguments.
116 expr1 * expr2 Multiplication of decimal integer-valued
117 arguments.
118 expr1 / expr2 Integer division of decimal integer-val‐
119 ued arguments, producing an integer
120 result.
121 expr1 % expr2 Remainder of integer division of decimal
122 integer-valued arguments.
123 expr1 : expr2 Matching expression; see below.
124 ( expr ) Grouping symbols. Any expression can be
125 placed within parentheses. Parentheses
126 can be nested to a depth of
127 {EXPR_NEST_MAX}.
128 integer An argument consisting only of an
129 (optional) unary minus followed by dig‐
130 its.
131 string A string argument; see below.
132 Matching Expression
134 The ':' matching operator shall compare the string resulting from the
135 evaluation of expr1 with the regular expression pattern resulting from
136 the evaluation of expr2. Regular expression syntax shall be that
137 defined in the Base Definitions volume of IEEE Std 1003.1-2001, Section
138 9.3, Basic Regular Expressions, except that all patterns are anchored
139 to the beginning of the string (that is, only sequences starting at the
140 first character of a string are matched by the regular expression) and,
141 therefore, it is unspecified whether '^' is a special character in that
142 context. Usually, the matching operator shall return a string repre‐
143 senting the number of characters matched ( '0' on failure). Alterna‐
144 tively, if the pattern contains at least one regular expression subex‐
145 pression "[\(...\)]" , the string corresponding to "\1" shall be
146 returned.
147 String Operand
149 A string argument is an argument that cannot be identified as an inte‐
150 ger argument or as one of the expression operator symbols shown in the
151 OPERANDS section.
152 The use of string arguments length, substr, index, or match produces
154 unspecified results.
155
157 The following exit values shall be returned:
158 0 The expression evaluates to neither null nor zero.
160 1 The expression evaluates to null or zero.
162 2 Invalid expression.
164 >2 An error occurred.
166
169 Default.
170 The following sections are informative.
172
174 After argument processing by the shell, expr is not required to be able
175 to tell the difference between an operator and an operand except by the
176 value. If "$a" is '=' , the command:
177 expr $a = '='
180 looks like:
182 expr = = =
185 as the arguments are passed to expr (and they all may be taken as the
187 '=' operator). The following works reliably:
188 expr X$a = X=
191 Also note that this volume of IEEE Std 1003.1-2001 permits implementa‐
193 tions to extend utilities. The expr utility permits the integer argu‐
194 ments to be preceded with a unary minus. This means that an integer
195 argument could look like an option. Therefore, the conforming applica‐
196 tion must employ the "--" construct of Guideline 10 of the Base Defini‐
197 tions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax
198 Guidelines to protect its operands if there is any chance the first op‐
199 erand might be a negative integer (or any string with a leading minus).
200
202 The expr utility has a rather difficult syntax:
203 * Many of the operators are also shell control operators or reserved
205 words, so they have to be escaped on the command line.
206 * Each part of the expression is composed of separate arguments, so
208 liberal usage of <blank>s is required. For example:
209 Invalid Valid
211 expr 1+2 expr 1 + 2
212 expr "1 + 2" expr 1 + 2
213 expr 1 + (2 * 3) expr 1 + \( 2 \* 3 \)
214 In many cases, the arithmetic and string features provided as part of
216 the shell command language are easier to use than their equivalents in
217 expr. Newly written scripts should avoid expr in favor of the new fea‐
218 tures within the shell; see Parameters and Variables and Arithmetic
219 Expansion .
220 The following command:
222 a=$(expr $a + 1)
225 adds 1 to the variable a.
227 The following command, for "$a" equal to either /usr/abc/file or just
229 file:
230 expr $a : '.*/\(.*\)' \| $a
233 returns the last segment of a pathname (that is, file). Applications
235 should avoid the character '/' used alone as an argument; expr may
236 interpret it as the division operator.
237 The following command:
239 expr "//$a" : '.*/\(.*\)'
242 is a better representation of the previous example. The addition of the
244 "//" characters eliminates any ambiguity about the division operator
245 and simplifies the whole expression. Also note that pathnames may con‐
246 tain characters contained in the IFS variable and should be quoted to
247 avoid having "$a" expand into multiple arguments.
248 The following command:
250 expr "$VAR" : '.*'
253 returns the number of characters in VAR.
255
257 In an early proposal, EREs were used in the matching expression syntax.
258 This was changed to BREs to avoid breaking historical applications.
259 The use of a leading circumflex in the BRE is unspecified because many
261 historical implementations have treated it as a special character,
262 despite their system documentation. For example:
263 expr foo : ^foo expr ^foo : ^foo
266 return 3 and 0, respectively, on those systems; their documentation
268 would imply the reverse. Thus, the anchoring condition is left unspeci‐
269 fied to avoid breaking historical scripts relying on this undocumented
270 feature.
271
273 None.
274
276 Parameters and Variables , Arithmetic Expansion
277
279 Portions of this text are reprinted and reproduced in electronic form
280 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
281 -- Portable Operating System Interface (POSIX), The Open Group Base
282 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
283 Electrical and Electronics Engineers, Inc and The Open Group. In the
284 event of any discrepancy between this version and the original IEEE and
285 The Open Group Standard, the original IEEE and The Open Group Standard
286 is the referee document. The original Standard can be obtained online
287 at http://www.opengroup.org/unix/online.html .
288IEEE/The Open Group 2003 EXPR(P)