EXPR(1) BSD General Commands Manual EXPR(1)NAMEexpr — evaluate expression
SYNOPSISexpr expression
DESCRIPTION
The expr utility evaluates expression and writes the result on standard
output.
All operators are separate arguments to the expr utility. Characters
special to the command interpreter must be escaped.
Operators are listed below in order of increasing precedence. Operators
with equal precedence are grouped within { } symbols.
expr1 | expr2
Returns the evaluation of expr1 if it is neither an empty string
nor zero; otherwise, returns the evaluation of expr2.
expr1 & expr2
Returns the evaluation of expr1 if neither expression evaluates
to an empty string or zero; otherwise, returns zero.
expr1 {=, >, ≥, <, ≤, !=} expr2
Returns the results of integer comparison if both arguments are
integers; otherwise, returns the results of string comparison
using 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 {+, -} expr2
Returns the results of addition or subtraction of integer-valued
arguments.
expr1 {*, /, %} expr2
Returns the results of multiplication, integer division, or
remainder of integer-valued arguments.
expr1 : expr2
The “:” operator matches expr1 against expr2, which must be a
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.
( expr )
Parentheses are used for grouping in the usual manner.
Additionally, the following keywords are recognized:
length expr
Returns the length of the specified string in bytes.
Operator precedence (from highest to lowest):
1. parentheses
2. length
3. “:”
4. “*”, “/”, and “%”
5. “+” and “-”
6. compare operators
7. “&”
8. “|”
EXIT STATUS
The expr utility 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.
>2 an error occurred (such as memory allocation failure).
EXAMPLES
1. The following example adds one to the variable a.
a=`expr $a + 1`
2. The following example returns zero, due to subtraction having higher
precedence than '&' operator.
expr 1 '&' 1 - 1
3. The following example returns the filename portion of a pathname
stored in variable a.
expr /$a : '.*/\(.*\)'
4. The following example returns the number of characters in variable
a.
expr $a : '.*'
COMPATIBILITY
This implementation of expr internally uses 64 bit representation of
integers and checks for over- and underflows. It also treats / (division
mark) and option '--' correctly depending upon context.
expr on other systems (including NetBSD up to and including NetBSD 1.5)
might not be so graceful. Arithmetic results might be arbitrarily lim‐
ited on such systems, most commonly to 32 bit quantities. This means
such expr can only process values between -2147483648 and +2147483647.
On other systems, expr might also not work correctly for regular expres‐
sions where either side contains single forward slash, like this:
expr / : '.*/\(.*\)'
If this is the case, you might use // (double forward slash) to avoid
confusion with the division operator:
expr "//$a" : '.*/\(.*\)'
According to IEEE Std 1003.2 (“POSIX.2”), expr has to recognize special
option '--', treat it as an end of command line options and ignore it.
Some expr implementations don't recognize it at all, others might ignore
it even in cases where doing so results in syntax error. There should be
same result for both following examples, but it might not always be:
1. expr-- : .
2. expr-- -- : .
Although NetBSD expr handles both cases correctly, you should not depend
on this behavior for portability reasons and avoid passing bare '--' as
first argument.
STANDARDS
The expr utility conforms to IEEE Std 1003.2 (“POSIX.2”). The length
keyword is an extension for compatibility with GNU expr.
AUTHORS
Original implementation was written by J.T. Conklin ⟨jtc@NetBSD.org⟩. It
was rewritten for NetBSD 1.6 by
Jaromir Dolecek ⟨jdolecek@NetBSD.org⟩.
NOTES
The empty string “” cannot be matched with the intuitive:
expr '' : '$'
The reason is that the returned number of matched characters (zero) is
indistinguishable from a failed match, so this returns failure. To match
the empty string, use something like:
expr x'' : 'x$'
BSD April 20, 2004 BSD
