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 "=".

Do not confuse the "=" assignment operator with the = test operator. # = as a test operator if [ "$string1" = "$string2" ] # if [ "X$string1" = "X$string2" ] 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 9-23 and Example 9-25) and formatting program output (see Example 26-8 and Example A-7). It can even be used to generate prime numbers, (see Example A-17). Modulo turns up surprisingly often in various numerical recipes.

Example 8-1. Greatest common divisor

#!/bin/bash # gcd.sh: greatest common divisor # Uses Euclid's algorithm # The "greatest common divisor" (gcd) of two integers #+ is the largest integer that will divide both, leaving no remainder. # Euclid's algorithm uses successive division. # In each pass, #+ dividend <--- divisor #+ divisor <--- remainder #+ until remainder = 0. #+ The gcd = dividend, on the final pass. # # For an excellent discussion of Euclid's algorithm, see # Jim Loy's site, http://www.jimloy.com/number/euclids.htm. # ------------------------------------------------------ # Argument check ARGS=2 E_BADARGS=65 if [ $# -ne "$ARGS" ] then echo "Usage: `basename $0` first-number second-number" exit $E_BADARGS fi # ------------------------------------------------------ gcd () { # Arbitrary assignment. dividend=$1 # It does not matter divisor=$2 #+ which of the two is larger. # Why? remainder=1 # If uninitialized variable used in loop, #+ it results in an error message #+ on first pass through loop. until [ "$remainder" -eq 0 ] do let "remainder = $dividend % $divisor" dividend=$divisor # Now repeat with 2 smallest numbers. divisor=$remainder done # Euclid's algorithm } # Last $dividend is the gcd. gcd $1 $2 echo; echo "GCD of $1 and $2 = $dividend"; echo # Exercise : # -------- # Check command-line arguments to make sure they are integers, #+ and exit the script with an appropriate error message if not. exit 0

+=

"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 8-2. 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

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.

&& 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.

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

Example 8-3. 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 10-12.