EXPR(1L) MachTen Programmer’s Manual EXPR(1L)

NAME
expr - evaluate expressions

SYNOPSIS
expr expression...
expr {--help,--version}

DESCRIPTION
This manual page documents the GNU version of expr. expr
evaluates an expression and writes the result on its stan-
dard output. Each token of the expression must be a sepa-
rate argument. Operands are either numbers or strings.
Strings are not quoted for expr, though you may need to
quote them to protect them from the shell. expr coerces
anything appearing in an operand position to an integer or
a string depending on the operation being applied to it.

The operators (in order of increasing precedence) are:

| Yields its first argument if it is neither null nor
0, otherwise its second argument. This is the
usual ‘or’ operation.

& Yields its first argument if neither argument is
null or 0, otherwise 0.

< <= = == != >= >
Compare their arguments and return 1 if the rela-
tion is true, 0 otherwise. (== is a synonym for
=.) expr tries to coerce both arguments to numbers
and do a numeric comparison; if it fails when try-
ing to coerce either argument it then does a lexi-
cographic comparison.

+ - Perform arithmetic operations. Both arguments are
coerced to numbers; an error occurs if this cannot
be done.

* / %
Perform arithmetic operations (‘%’ is the remainder
operation, as in C). Both arguments are coerced to
numbers; an error occurs if this cannot be done.

: Perform pattern matching. Its arguments are
coerced to strings and the second one is considered
to be a regular expression, with a ‘^’ implicitly
added at the beginning. The first argument is then
matched against this regular expression. If the
match succeeds and part of the string is enclosed
in ‘and ‘’, that part is the value of the :
expression; otherwise an integer whose value is the
number of characters matched is returned. If the
match fails, the : operator returns the null string
if ‘and ‘’ are used, otherwise 0. Only one
‘and ‘’ pair can be used.

In addition, the following keywords are recognized:

match string regex
An alternative way to do pattern matching. This is
the same as ‘‘string : regex’’.

substr string position length
Return the substring of string beginning at posi-
tion with length at most length. If either posi-
tion or length is negative or non-numeric, return a
null string.

index string character-class
Return the first position in string where the first
character in character-class was found. If no
character in character-class is found in string,
return 0.

length string
Return the length of string.

Parentheses are used for grouping in the usual manner.
The keywords cannot be used as strings.

OPTIONS
When GNU expr is invoked with exactly one argument, the
following options are recognized:

--help Print a usage message on standard output and exit
successfully.

--version
Print version information on standard output then
exit successfully.

EXAMPLES
To add 1 to the shell variable a:

a=‘expr $a + 1‘

The following may be used to print the non-directory part
of the file name stored in variable a (the value in a need
not contain ‘/’):

expr $a : ’.*/’ ’|’ $a

Note the quoted shell metacharacters.

expr returns the following exit status:

0 if the expression is neither null nor 0,
1 if the expression is null or 0,
2 for invalid expressions.

FSF GNU Shell Utilities 2