- =
All-purpose assignment operator, which works for both arithmetic and
string assignments.
May also be used in a string comparison test.
if [ $string1 = $string2 ]
then
command
fi |
The following are normally used in combination with
expr or let.
arithmetic operators
- +
plus
- -
minus
- *
multiplication
- /
division
- %
modulo, or mod (returns the remainder of an integer
division)
- +=
"plus-equal" (increment variable by a constant)
`expr $var+=5` results in
var being incremented by
5.
- -=
"minus-equal" (decrement variable by a constant)
- *=
"times-equal" (multiply variable by a constant)
`expr $var*=4` results in var
being multiplied by 4.
- /=
"slash-equal" (divide variable by a constant)
- %=
"mod-equal" (remainder of dividing variable by a constant)
The bitwise logical operators seldom make an appearance in shell scripts.
Their chief use seems to be manipulating and testing values read from
ports or sockets. "Bit flipping" is more relevant to compiled languages,
such as C and C++, which run fast enough to permit its use on the fly.
- <<
bitwise left shift (multiplies by 2
for each shift position)
- <<=
"left-shift-equal"
let "var <<= 2" results in var
left-shifted 2 bits (multiplied by 4)
- >>
bitwise right shift (divides by 2
for each shift position)
- >>=
"right-shift-equal" (inverse of <<=)
- &
bitwise and
- &=
"bitwise and-equal"
- |
bitwise OR
- |=
"bitwise OR-equal"
- ~
bitwise negate
- !
bitwise NOT
- ^
bitwise XOR
- ^=
"bitwise XOR-equal"
relational tests
- <
less than
- >
greater than
- <=
less than or equal to
- >=
greater than or equal to
- ==
equal to (test)
- !=
not equal to
- &&
and (logical)
if [ $condition1 ] && [ $condition2 ]
# if both condition1 and condition2 hold true... |
Note: && may also, depending on context, be
used to in an and list to concatenate
commands (see Section 3.21).
- ||
or (logical)
if [ $condition1 ] || [ $condition2 ]
# if both condition1 or condition2 hold true... |
Example 3-15. Compound Condition Tests Using && and ||
#!/bin/bash
a=24
b=47
if [ $a -eq 24 ] && [ $b -eq 47 ]
then
echo "Test #1 succeeds."
else
echo "Test #1 fails."
fi
# ERROR:
# if [ $a -eq 24 && $b -eq 47 ]
if [ $a -eq 98 ] || [ $b -eq 47 ]
then
echo "Test #2 succeeds."
else
echo "Test #2 fails."
fi
# The -a and -o options provide
# an alternative compound condition test.
# Thanks to Patrick Callahan for pointing this out.
if [ $a -eq 24 -a $b -eq 47 ]
then
echo "Test #3 succeeds."
else
echo "Test #3 fails."
fi
if [ $a -eq 98 -o $b -eq 47 ]
then
echo "Test #4 succeeds."
else
echo "Test #4 fails."
fi
a=rhino
b=crocodile
if [ $a = rhino ] && [ $b = crocodile ]
then
echo "Test #5 succeeds."
else
echo "Test #5 fails."
fi
exit 0 |
A shell script interprets a number as decimal (base 10), unless
that number has a special prefix or notation. A number preceded by a
0 is octal (base
8). A number preceded by 0x is
hexadecimal (base 16). A number with an
embedded # is evaluated as
BASE#NUMBER (this option is of limited
usefulness because of range restrictions).
Example 3-16. Representation of numerical constants:
#!/bin/bash
# Representation of numbers.
# Decimal
let "d = 32"
echo "d = $d"
# Nothing out of the ordinary here.
# Octal: numbers preceded by '0'
let "o = 071"
echo "o = $o"
# Expresses result in decimal.
# Hexadecimal: numbers preceded by '0x' or '0X'
let "h = 0x7a"
echo "h = $h"
# Other bases: BASE#NUMBER
# BASE between 2 and 64.
let "b = 32#77"
echo "b = $b"
# This notation only works for a very limited range of numbers.
let "c = 2#47" # Error: out of range.
echo "c = $c"
exit 0 |