let, another
command unique to POSIX and Korn Shell, enables shell scripts to
use arithmetic expressions. This command allows long integer arithmetic.
The syntax is:
where each arg is an arithmetic
expression of shell parameters and operators to be evaluated by
the shell. Table 21-2 “Operator Decreasing Precedence Order” lists the
operators in decreasing order of precedence.
Table 21-2 Operator Decreasing Precedence Order
Operator | Description |
|---|
- | unary minus |
! | logical negation |
* / % | multiplication, division, remainder |
+ - | addition, subtraction |
<= >= < > | comparison |
== != | equals, does not equal |
= | assignment |
In the next example, x
is set to 1, and then when the let
command executes:
first, 1*6 is evaluated to 6,
then 3/1 is evaluated to 3,
then x
is added to the 6 which equals 7, and
finally the 3 is subtracted from the 7 to equal
4.
$ x=1
$ let x=x+1*6-3/1
$ echo $x
4 |
You can also use parenthesis to create this effect, or override
the operator's precedence to produce different results, 9. (When
using parenthesis, double quotes are necessary.)
$ let "x=x+(1*6)-(3/1)"
$ let "x=(x+1)*6-3/1" |
This next script reads a value from the user, compares it
to 14, and prints an appropriate string based on the comparison:
$ read x
$ y=14
$ if (( x >= y ))
> then echo greater or equal
> else echo less than
> fi |
Using (( ))
around the expression replaces using the let:
(which must be quoted to allow blanks and prevent the >
from being interpreted as an I/O redirection). Also, you do not
need to put $
in front of x
or y. In this
situation, the let
command is used as a condition.
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