Advanced Bash-Scripting HOWTO: A guide to shell scripting, using Bash | ||
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- =
All-purpose assignment operator, which works for both arithmetic and string assignments.

var=27 category=minerals

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)

results in**`expr $var+=5`**`var`being incremented by`5`.- -=
"minus-equal" (decrement variable by a constant)

- *=
"times-equal" (multiply variable by a constant)

results in**`expr $var*=4`**`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"

results in**let "var <<= 2"**`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...

- ||
or (logical)

if [ $condition1 ] || [ $condition2 ] # if both condition1 or condition2 hold true...

**Example 3-14. 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

**Example 3-15. 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 |

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