Originální popis anglicky:
expr - evaluate arguments as an expression
Návod, kniha: POSIX Programmer's Manual
expr operand
The
expr utility shall evaluate an expression and write the result to
standard output.
None.
The single expression evaluated by
expr shall be formed from the
operands, as described in the EXTENDED DESCRIPTION section. The application
shall ensure that each of the expression operator symbols:
( ) | & = > >= < <= != + - * / % :
and the symbols
integer and
string in the table are provided as
separate arguments to
expr.
Not used.
None.
The following environment variables shall affect the execution of
expr:
- LANG
- Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization
Variables for the precedence of internationalization variables used to
determine the values of locale categories.)
- LC_ALL
- If set to a non-empty string value, override the values of
all the other internationalization variables.
- LC_COLLATE
-
Determine the locale for the behavior of ranges, equivalence classes, and
multi-character collating elements within regular expressions and by the
string comparison operators.
- LC_CTYPE
- Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments) and the behavior of character classes
within regular expressions.
- LC_MESSAGES
- Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
- NLSPATH
- Determine the location of message catalogs for the
processing of LC_MESSAGES .
Default.
The
expr utility shall evaluate the expression and write the result,
followed by a <newline>, to standard output.
The standard error shall be used only for diagnostic messages.
None.
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 shall be left-associative.
| Expression |
Description |
| expr1 | expr2 |
Returns the evaluation of expr1 if it is neither null nor zero;
otherwise, returns the evaluation of expr2 if it is not null;
otherwise, zero. |
| expr1 & expr2 |
Returns the evaluation of expr1 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 is 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 {EXPR_NEST_MAX}. |
| integer |
An argument consisting only of an (optional) unary minus followed by
digits. |
| string |
A string argument; see below. |
The
':' matching operator shall compare the string resulting from the
evaluation of
expr1 with the regular expression pattern resulting from
the evaluation of
expr2. Regular expression syntax shall be that
defined in the Base Definitions volume of IEEE Std 1003.1-2001,
Section 9.3, 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 are matched by the regular expression) and, therefore,
it is unspecified whether
'^' is a special character in that context.
Usually, the matching operator shall 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" shall be returned.
A 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 shall be returned:
- 0
- The expression evaluates to neither null nor
zero.
- 1
- The expression evaluates to null or zero.
- 2
- Invalid expression.
- >2
- An error occurred.
Default.
The following sections are informative.
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:
as the arguments are passed to
expr (and they all may be taken as the
'=' operator). The following works reliably:
Also note that this volume of IEEE Std 1003.1-2001 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 conforming
application must employ the
"--" construct of Guideline 10 of
the Base Definitions volume of IEEE Std 1003.1-2001, Section
12.2, 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>s 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 favor of the new features
within the shell; see
Parameters and Variables and
Arithmetic Expansion .
The following command:
adds 1 to the variable
a.
The following command, for
"$a" equal to either
/usr/abc/file or just
file:
expr $a : '.*/\(.*\)' \| $a
returns the last segment of a pathname (that is,
file). Applications
should avoid the character
'/' used alone as an argument;
expr
may interpret it as the division operator.
The following command:
expr "//$a" : '.*/\(.*\)'
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.
The following command:
returns the number of characters in
VAR.
In an early proposal, EREs were used in the matching expression syntax. This was
changed to BREs to avoid breaking historical applications.
The use of a leading circumflex in the BRE is unspecified because many
historical implementations have treated it as a special character, despite
their system documentation. For example:
expr foo : ^foo expr ^foo : ^foo
return 3 and 0, respectively, on those systems; their documentation would imply
the reverse. Thus, the anchoring condition is left unspecified to avoid
breaking historical scripts relying on this undocumented feature.
None.
Parameters and Variables ,
Arithmetic Expansion
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
