1EXPR(1P) POSIX Programmer's Manual EXPR(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 expr — evaluate arguments as an expression
13
15 expr operand...
16
18 The expr utility shall evaluate an expression and write the result to
19 standard output.
20
22 None.
23
25 The single expression evaluated by expr shall be formed from the oper‐
26 and operands, as described in the EXTENDED DESCRIPTION section. The
27 application shall ensure that each of the expression operator symbols:
28 ( ) | & = > >= < <= != + - * / % :
31 and the symbols integer and string in the table are provided as sepa‐
33 rate arguments to expr.
34
36 Not used.
37
39 None.
40
42 The following environment variables shall affect the execution of expr:
43 LANG Provide a default value for the internationalization vari‐
45 ables that are unset or null. (See the Base Definitions vol‐
46 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
47 ables for the precedence of internationalization variables
48 used to determine the values of locale categories.)
49 LC_ALL If set to a non-empty string value, override the values of
51 all the other internationalization variables.
52 LC_COLLATE
54 Determine the locale for the behavior of ranges, equivalence
55 classes, and multi-character collating elements within regu‐
56 lar expressions and by the string comparison operators.
57 LC_CTYPE Determine the locale for the interpretation of sequences of
59 bytes of text data as characters (for example, single-byte as
60 opposed to multi-byte characters in arguments) and the behav‐
61 ior of character classes within regular expressions.
62 LC_MESSAGES
64 Determine the locale that should be used to affect the format
65 and contents of diagnostic messages written to standard
66 error.
67 NLSPATH Determine the location of message catalogs for the processing
69 of LC_MESSAGES.
70
72 Default.
73
75 The expr utility shall evaluate the expression and write the result,
76 followed by a <newline>, to standard output.
77
79 The standard error shall be used only for diagnostic messages.
80
82 None.
83
85 The formation of the expression to be evaluated is shown in the follow‐
86 ing table. The symbols expr, expr1, and expr2 represent expressions
87 formed from integer and string symbols and the expression operator sym‐
88 bols (all separate arguments) by recursive application of the con‐
89 structs described in the table. The expressions are listed in order of
90 decreasing precedence, with equal-precedence operators grouped between
91 horizontal lines. All of the operators shall be left-associative.
92 ┌───────────────┬──────────────────────────────────────────┐
94 │ Expression │ Description │
95 ├───────────────┼──────────────────────────────────────────┤
96 │integer │ An argument consisting only of an │
97 │ │ (optional) unary minus followed by dig‐ │
98 │ │ its. │
99 │string │ A string argument; see below. │
100 ├───────────────┼──────────────────────────────────────────┤
101 │( expr ) │ Grouping symbols. Any expression can be │
102 │ │ placed within parentheses. Parentheses │
103 │ │ can be nested to a depth of │
104 │ │ {EXPR_NEST_MAX}. │
105 ├───────────────┼──────────────────────────────────────────┤
106 │expr1 : expr2 │ Matching expression; see below. │
107 ├───────────────┼──────────────────────────────────────────┤
108 │expr1 * expr2 │ Multiplication of decimal integer-valued │
109 │ │ arguments. │
110 │expr1 / expr2 │ Integer division of decimal integer-val‐ │
111 │ │ ued arguments, producing an integer │
112 │ │ result. │
113 │expr1 % expr2 │ Remainder of integer division of decimal │
114 │ │ integer-valued arguments. │
115 ├───────────────┼──────────────────────────────────────────┤
116 │expr1 + expr2 │ Addition of decimal integer-valued argu‐ │
117 │ │ ments. │
118 │expr1 - expr2 │ Subtraction of decimal integer-valued │
119 │ │ arguments. │
120 ├───────────────┼──────────────────────────────────────────┤
121 │ │ Returns the result of a decimal integer │
122 │ │ comparison if both arguments are inte‐ │
123 │ │ gers; otherwise, returns the result of a │
124 │ │ string comparison using the locale-spe‐ │
125 │ │ cific collation sequence. The result of │
126 │ │ each comparison is 1 if the specified │
127 │ │ relationship is true, or 0 if the rela‐ │
128 │ │ tionship is false. │
129 │expr1 = expr2 │ Equal. │
130 │expr1 > expr2 │ Greater than. │
131 │expr1 >= expr2 │ Greater than or equal. │
132 │expr1 < expr2 │ Less than. │
133 │expr1 <= expr2 │ Less than or equal. │
134 │expr1 != expr2 │ Not equal. │
135 ├───────────────┼──────────────────────────────────────────┤
136 │expr1 & expr2 │ Returns the evaluation of expr1 if nei‐ │
137 │ │ ther expression evaluates to null or │
138 │ │ zero; otherwise, returns zero. │
139 ├───────────────┼──────────────────────────────────────────┤
140 │expr1 | expr2 │ Returns the evaluation of expr1 if it is │
141 │ │ neither null nor zero; otherwise, │
142 │ │ returns the evaluation of expr2 if it is │
143 │ │ not null; otherwise, zero. │
144 └───────────────┴──────────────────────────────────────────┘
145 Matching Expression
146 The ':' matching operator shall compare the string resulting from the
147 evaluation of expr1 with the regular expression pattern resulting from
148 the evaluation of expr2. Regular expression syntax shall be that
149 defined in the Base Definitions volume of POSIX.1‐2017, Section 9.3,
150 Basic Regular Expressions, except that all patterns are anchored to the
151 beginning of the string (that is, only sequences starting at the first
152 character of a string are matched by the regular expression) and,
153 therefore, it is unspecified whether '^' is a special character in that
154 context. Usually, the matching operator shall return a string repre‐
155 senting the number of characters matched ('0' on failure). Alterna‐
156 tively, if the pattern contains at least one regular expression subex‐
157 pression "[\(...\)]", the string matched by the back-reference expres‐
158 sion "\1" shall be returned. If the back-reference expression "\1" does
159 not match, then the null string shall be returned.
160 Identification as Integer or String
162 An argument or the value of a subexpression that consists only of an
163 optional unary minus followed by digits is a candidate for treatment as
164 an integer if it is used as the left argument to the | operator or as
165 either argument to any of the following operators: & = > >= < <= != + -
166 * / %. Otherwise, the argument or subexpression value shall be treated
167 as a string.
168 The use of string arguments length, substr, index, or match produces
170 unspecified results.
171
173 The following exit values shall be returned:
174 0 The expression evaluates to neither null nor zero.
176 1 The expression evaluates to null or zero.
178 2 Invalid expression.
180 >2 An error occurred.
182
184 Default.
185 The following sections are informative.
187
189 The expr utility has a rather difficult syntax:
190 * Many of the operators are also shell control operators or reserved
192 words, so they have to be escaped on the command line.
193 * Each part of the expression is composed of separate arguments, so
195 liberal usage of <blank> characters is required. For example:
196 ┌─────────────────┬───────────────────────┐
198 │ Invalid │ Valid │
199 ├─────────────────┼───────────────────────┤
200 │expr 1+2 │ expr 1 + 2 │
201 │expr "1 + 2" │ expr 1 + 2 │
202 │expr 1 + (2 * 3) │ expr 1 + \( 2 \* 3 \) │
203 └─────────────────┴───────────────────────┘
204 In many cases, the arithmetic and string features provided as part of
205 the shell command language are easier to use than their equivalents in
206 expr. Newly written scripts should avoid expr in favor of the new fea‐
207 tures within the shell; see Section 2.5, Parameters and Variables and
208 Section 2.6.4, Arithmetic Expansion.
209 After argument processing by the shell, expr is not required to be able
211 to tell the difference between an operator and an operand except by the
212 value. If "$a" is '=', the command:
213 expr "$a" = '='
216 looks like:
218 expr = = =
221 as the arguments are passed to expr (and they all may be taken as the
223 '=' operator). The following works reliably:
224 expr "X$a" = X=
227 Also note that this volume of POSIX.1‐2017 permits implementations to
229 extend utilities. The expr utility permits the integer arguments to be
230 preceded with a unary minus. This means that an integer argument could
231 look like an option. Therefore, the conforming application must employ
232 the "--" construct of Guideline 10 of the Base Definitions volume of
233 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines to protect its
234 operands if there is any chance the first operand might be a negative
235 integer (or any string with a leading minus).
236 For testing string equality the test utility is preferred over expr, as
238 it is usually implemented as a shell built-in. However, the functional‐
239 ity is not quite the same because the expr = and != operators check
240 whether strings collate equally, whereas test checks whether they are
241 identical. Therefore, they can produce different results in locales
242 where the collation sequence does not have a total ordering of all
243 characters (see the Base Definitions volume of POSIX.1‐2017, Section
244 7.3.2, LC_COLLATE).
245
247 The following command:
248 a=$(expr "$a" + 1)
251 adds 1 to the variable a.
253 The following command, for "$a" equal to either /usr/abc/file or just
255 file:
256 expr $a : '.*/\(.*\)' \| $a
259 returns the last segment of a pathname (that is, file). Applications
261 should avoid the character '/' used alone as an argument; expr may
262 interpret it as the division operator.
263 The following command:
265 expr "//$a" : '.*/\(.*\)'
268 is a better representation of the previous example. The addition of the
270 "//" characters eliminates any ambiguity about the division operator
271 and simplifies the whole expression. Also note that pathnames may con‐
272 tain characters contained in the IFS variable and should be quoted to
273 avoid having "$a" expand into multiple arguments.
274 The following command:
276 expr "X$VAR" : '.*' - 1
279 returns the number of characters in VAR.
281
283 In an early proposal, EREs were used in the matching expression syntax.
284 This was changed to BREs to avoid breaking historical applications.
285 The use of a leading <circumflex> in the BRE is unspecified because
287 many historical implementations have treated it as a special character,
288 despite their system documentation. For example:
289 expr foo : ^foo expr ^foo : ^foo
292 return 3 and 0, respectively, on those systems; their documentation
294 would imply the reverse. Thus, the anchoring condition is left unspeci‐
295 fied to avoid breaking historical scripts relying on this undocumented
296 feature.
297
299 None.
300
302 Section 2.5, Parameters and Variables, Section 2.6.4, Arithmetic Expan‐
303 sion
304 The Base Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE,
306 Chapter 8, Environment Variables, Section 9.3, Basic Regular Expres‐
307 sions, Section 12.2, Utility Syntax Guidelines
308
310 Portions of this text are reprinted and reproduced in electronic form
311 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
312 table Operating System Interface (POSIX), The Open Group Base Specifi‐
313 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
314 Electrical and Electronics Engineers, Inc and The Open Group. In the
315 event of any discrepancy between this version and the original IEEE and
316 The Open Group Standard, the original IEEE and The Open Group Standard
317 is the referee document. The original Standard can be obtained online
318 at http://www.opengroup.org/unix/online.html .
319 Any typographical or formatting errors that appear in this page are
321 most likely to have been introduced during the conversion of the source
322 files to man page format. To report such errors, see https://www.ker‐
323 nel.org/doc/man-pages/reporting_bugs.html .
324IEEE/The Open Group 2017 EXPR(1P)