Section: User Commands (1)
Updated: GNU Shell UtilitiesIndex
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expr - evaluate expressions
This documentation is no longer being maintained and may be inaccurate
or incomplete. The Texinfo documentation is now the authoritative source.
This manual page
documents the GNU version of
evaluates an expression and writes the result on its standard output.
Each token of the expression must be a separate 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.
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,
- < <= = == != >= >
Compare their arguments and return 1 if the relation 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 trying to coerce either
argument it then does a lexicographic 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 position with
length at most length. If either position 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.
is invoked with exactly one argument, the following options are recognized:
Print a usage message on standard output and exit successfully.
Print version information on standard output then exit successfully.
To add 1 to the shell variable
a=`expr $a + 1`
The following may be used to print the non-directory part of the file name stored in variable
(the value in
need not contain `/'):
expr $a : '.*/\(.*\)' '|' $a
Note the quoted shell metacharacters.
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.