EXPR(1)

EXPR(1) | General Commands Manual | EXPR(1)

# NAME

`expr` — evaluate expression

# SYNOPSIS

`expr` | operand ...

# DESCRIPTION

The `expr` utility evaluates the expression consisting of the operand arguments and writes the result on standard output.

Each operand is a separate argument 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 if it is not empty; otherwise, returns zero.
expr1 `&` expr2
  Returns the evaluation of expr1 if neither expression evaluates to an empty string or zero; otherwise, returns zero.
expr1 `{=, >, >=, <, <=, !=}` expr2
  Returns the result of the integer comparison if both arguments are integers; otherwise, returns the result of the 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 result of addition or subtraction of integer-valued arguments.
expr1 `{*, /, %}` expr2
  Returns the result of multiplication, integer division, or remainder of integer-valued arguments.
expr1 `:` expr2
  The “:” operator matches expr1 against expr2, the latter 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 regular 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 empty 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 variable “a”:
   `a=`expr $a + 1``
2. The following example returns the filename portion of a pathname stored in variable “a”:
   `expr /$a : `'.*/\(.*\)'``
3. 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 “/” (the 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 limited 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 expressions where either side contains “/” (a single forward slash), like this:

    expr / : '.*/\(.*\)'

If this is the case, you might use “//” (a double forward slash) to avoid confusion with the division operator:

    expr "//$a" : '.*/\(.*\)'

According to IEEE Std 1003.2 (“POSIX.2”), `expr` has to recognize the special option “--”, treat it as a delimiter to mark the end of command line options, and ignore it. Some `expr` implementations do not recognize it at all; others might ignore it even in cases where doing so results in syntax error. There should be the 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 a bare “--” as the 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`.

# HISTORY

An `expr` utility first appeared in the Programmer's Workbench (PWB/UNIX). A public domain version of `expr` written by Pace Willisson ⟨pace@blitz.com⟩ appeared in 386BSD-0.1.

# AUTHORS

Initial implementation by Pace Willisson <pace@blitz.com> was largely rewritten by J.T. Conklin <jtc@NetBSD.org>. It was rewritten again 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$'

June 28, 2025 | NetBSD 11.0