NAME

expr − evaluate arguments as an expression

SYNOPSIS

expr arguments

DESCRIPTION

The arguments are taken as an expression.  After evaluation, the result is written on the standard output.  Terms of the expression must be separated by blanks.  Characters special to the shell must be escaped.  Note that 0 is returned to indicate a zero value, rather than the null string.  Strings containing blanks or other special characters should be quoted.  Integer-valued arguments may be preceded by a unary minus sign.  Internally, integers are treated as 32-bit, 2’s complement numbers. 

The operators and keywords are listed below.  Characters that need to be escaped are preceded by \.  The list is in order of increasing precedence, with equal precedence operators grouped within {} symbols. 

expr \│ expr
returns the first expr if it is neither null nor 0, otherwise returns the second expr.

expr \& expr
returns the first expr if neither expr is null or 0, otherwise returns 0. 

expr { =, \>, \>=, \<, \<=, != } expr
returns the result of an integer comparison if both arguments are integers, otherwise returns the result of a lexical comparison.

expr { +, − } expr
addition or subtraction of integer-valued arguments.

expr { \∗, /, % } expr
multiplication, division, or remainder of the integer-valued arguments.

expr : expr
The matching operator : compares the first argument with the second argument which must be a regular expression; regular expression syntax is the same as that of ed(1), except that all patterns are “anchored” (i.e., begin with ^) and, therefore, ^ is not a special character, in that context.  Normally, the matching operator returns the number of characters matched (0 on failure).  Alternatively, the \(...\) pattern symbols can be used to return a portion of the first argument. 

EXAMPLES

1.  a=`expr $a + 1`

adds 1 to the shell variable a. 

2.  #  ´For $a equal to either "/usr/abc/file" or just "file"´
expr  $a  :  ´.∗/\(.∗\)´  \│  $a

returns the last segment of a path name (i.e., file).  Watch out for / alone as an argument: expr will take it as the division operator (see BUGS below). 

3.  #  A better representation of example 2. 
expr  //$a  :  ´.∗/\(.∗\)´

The addition of the // characters eliminates any ambiguity about the division operator and simplifies the whole expression. 

4.  expr  $VAR  :  ´.∗´

returns the number of characters in $VAR. 

SEE ALSO

ed(1), sh(1). 

EXIT CODE

As a side effect of expression evaluation, expr returns the following exit values:
0if the expression is neither null nor 0
1if the expression is null or 0
2for invalid expressions. 

DIAGNOSTICS

syntax error for operator/operand errors

non-numeric argument
if arithmetic is attempted on such a string

BUGS

After argument processing by the shell, expr cannot tell the difference between an operator and an operand except by the value.  If $a is an =, the command:

expr  $a  =  ´=´

looks like:

expr  =  =  =

as the arguments are passed to expr (and they will all be taken as the = operator).  The following works:

expr  X$a  =  X=

May 16, 1980