8.1. Operators

assignment

variable assignment

Initializing or changing the value of a variable

=

All-purpose assignment operator, which works for both arithmetic and string assignments.

var=27
category=minerals  # No spaces allowed after the "=".

Caution

Do not confuse the "=" assignment operator with the = test operator.

#    = as a test operator

if [ "$string1" = "$string2" ]
# if [ "Xstring1" = "Xstring2" ] is safer,
# to prevent an error message should one of the variables be empty.
# (The prepended "X" characters cancel out.) 
then
   command
fi

arithmetic operators

+

plus

-

minus

*

multiplication

/

division

**

exponentiation
# Bash, version 2.02, introduced the "**" exponentiation operator.

let "z=5**3"
echo "z = $z"   # z = 125

%

modulo, or mod (returns the remainder of an integer division operation)

bash$ echo `expr 5 % 3`
2
	      

This operator finds use in, among other things, generating numbers within a specific range (see Example 9-20 and Example 9-21) and formatting program output (see Example 26-6). It can even be used to generate prime numbers, (see Example A-11).

+=

"plus-equal" (increment variable by a constant)

let "var += 5" results in var being incremented by 5.

-=

"minus-equal" (decrement variable by a constant)

*=

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

let "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)

Arithmetic operators often occur in an expr or let expression.

Example 8-1. Using Arithmetic Operations

#!/bin/bash
# Counting to 6 in 5 different ways.

n=1; echo -n "$n "

let "n = $n + 1"   # let "n = n + 1"   also works.
echo -n "$n "

: $((n = $n + 1))
#  ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo -n "$n "

n=$(($n + 1))
echo -n "$n "

: $[ n = $n + 1 ]
#  ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
# Works even if "n" was initialized as a string.
echo -n "$n "

n=$[ $n + 1 ]
#  Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
echo -n "$n "; echo

# Thanks, Stephane Chazelas.

exit 0

Note

Integer variables in Bash are actually signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that takes a variable outside these limits will give an erroneous result.
a=2147483646
echo "a = $a"      # a = 2147483646
let "a+=1"         # Increment "a".
echo "a = $a"      # a = 2147483647
let "a+=1"         # increment "a" again, past the limit.
echo "a = $a"      # a = -2147483648
                   #      ERROR (out of range)

Caution

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings.
a=1.5

let "b = $a + 1.3"  # Error.
# t2.sh: let: b = 1.5 + 1.3: syntax error in expression (error token is ".5 + 1.3")

echo "b = $b"       # b=1
Use bc in scripts that that need floating point calculations or math library functions.

bitwise operators. The bitwise 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 operators

<<

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"

logical operators

&&

and (logical)

if [ $condition1 ] && [ $condition2 ]
# Same as:  if [ $condition1 -a $condition2 ]
# Returns true if both condition1 and condition2 hold true...

if [[ $condition1 && $condition2 ]]    # Also works.
# Note that && operator not permitted within [ ... ] construct.

Note

&& may also, depending on context, be used in an and list to concatenate commands.

||

or (logical)

if [ $condition1 ] || [ $condition2 ]
# Same as:  if [ $condition1 -o $condition2 ]
# Returns true if either condition1 or condition2 holds true...

if [[ $condition1 || $condition2 ]]    # Also works.
# Note that || operator not permitted within [ ... ] construct.

Note

Bash tests the exit status of each statement linked with a logical operator.

Example 8-2. 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 ]
#          attempts to execute  ' [ "$a" -eq 24 '
#          and fails to finding matching ']'.
#
#    if [[ $a -eq 24 && $b -eq 24 ]]   works
#    (The "&&" has a different meaning in line 17 than in line 6.)
#    Thanks, Stephane Chazelas.


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

The && and || operators also find use in an arithmetic context.

bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))
1 0 1 0
	      

miscellaneous operators

,

comma operator

The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects, but only the last operation is returned.

let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
echo "t1 = $t1"               # t1 = 11

let "t2 = ((a = 9, 15 / 3))"  # Set "a" and calculate "t2".
echo "t2 = $t2    a = $a"     # t2 = 5    a = 9

The comma operator finds use mainly in for loops. See Example 10-11.