assignment

variable assignment

Initializing or changing the value of a variable

=

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

1 var=27 2 category=minerals # No spaces allowed after the "=".

Do not confuse the "=" assignment operator with the = test operator. 1 # = as a test operator 2  3 if [ "$string1" = "$string2" ] 4 then 5  command 6 fi 7  8 # if [ "X$string1" = "X$string2" ] is safer, 9 #+ to prevent an error message should one of the variables be empty. 10 # (The prepended "X" characters cancel out.)

arithmetic operators

+

plus

-

minus

*

multiplication

/

division

**

exponentiation

1 # Bash, version 2.02, introduced the "**" exponentiation operator. 2  3 let "z=5**3" 4 echo "z = $z" # z = 125

%

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

bash$ expr 5 % 3 2

5/3 = 1 with remainder 2

This operator finds use in, among other things, generating numbers within a specific range (see Example 9-25 and Example 9-28) and formatting program output (see Example 26-15 and Example A-6). It can even be used to generate prime numbers, (see Example A-16). Modulo turns up surprisingly often in various numerical recipes.

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Example 8-1. Greatest common divisor

1 #!/bin/bash 2 # gcd.sh: greatest common divisor 3 # Uses Euclid's algorithm 4  5 # The "greatest common divisor" (gcd) of two integers 6 #+ is the largest integer that will divide both, leaving no remainder. 7  8 # Euclid's algorithm uses successive division. 9 # In each pass, 10 #+ dividend <--- divisor 11 #+ divisor <--- remainder 12 #+ until remainder = 0. 13 #+ The gcd = dividend, on the final pass. 14 # 15 # For an excellent discussion of Euclid's algorithm, see 16 #+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm. 17  18  19 # ------------------------------------------------------ 20 # Argument check 21 ARGS=2 22 E_BADARGS=65 23  24 if [ $# -ne "$ARGS" ] 25 then 26  echo "Usage: `basename $0` first-number second-number" 27  exit $E_BADARGS 28 fi 29 # ------------------------------------------------------ 30  31  32 gcd () 33 { 34  35  dividend=$1 # Arbitrary assignment. 36  divisor=$2 #! It doesn't matter which of the two is larger. 37  # Why not? 38  39  remainder=1 # If uninitialized variable used in loop, 40  #+ it results in an error message 41  #+ on the first pass through loop. 42  43  until [ "$remainder" -eq 0 ] 44  do 45  let "remainder = $dividend % $divisor" 46  dividend=$divisor # Now repeat with 2 smallest numbers. 47  divisor=$remainder 48  done # Euclid's algorithm 49  50 } # Last $dividend is the gcd. 51  52  53 gcd $1 $2 54  55 echo; echo "GCD of $1 and $2 = $dividend"; echo 56  57  58 # Exercise : 59 # -------- 60 # Check command-line arguments to make sure they are integers, 61 #+ and exit the script with an appropriate error message if not. 62  63 exit 0

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+=

"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.

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Example 8-2. Using Arithmetic Operations

1 #!/bin/bash 2 # Counting to 11 in 10 different ways. 3  4 n=1; echo -n "$n " 5  6 let "n = $n + 1" # let "n = n + 1" also works. 7 echo -n "$n " 8  9  10 : $((n = $n + 1)) 11 # ":" necessary because otherwise Bash attempts 12 #+ to interpret "$((n = $n + 1))" as a command. 13 echo -n "$n " 14  15 (( n = n + 1 )) 16 # A simpler alternative to the method above. 17 # Thanks, David Lombard, for pointing this out. 18 echo -n "$n " 19  20 n=$(($n + 1)) 21 echo -n "$n " 22  23 : $[ n = $n + 1 ] 24 # ":" necessary because otherwise Bash attempts 25 #+ to interpret "$[ n = $n + 1 ]" as a command. 26 # Works even if "n" was initialized as a string. 27 echo -n "$n " 28  29 n=$[ $n + 1 ] 30 # Works even if "n" was initialized as a string. 31 #* Avoid this type of construct, since it is obsolete and nonportable. 32 # Thanks, Stephane Chazelas. 33 echo -n "$n " 34  35 # Now for C-style increment operators. 36 # Thanks, Frank Wang, for pointing this out. 37  38 let "n++" # let "++n" also works. 39 echo -n "$n " 40  41 (( n++ )) # (( ++n ) also works. 42 echo -n "$n " 43  44 : $(( n++ )) # : $(( ++n )) also works. 45 echo -n "$n " 46  47 : $[ n++ ] # : $[ ++n ]] also works 48 echo -n "$n " 49  50 echo 51  52 exit 0

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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)

1 if [ $condition1 ] && [ $condition2 ] 2 # Same as: if [ $condition1 -a $condition2 ] 3 # Returns true if both condition1 and condition2 hold true... 4  5 if [[ $condition1 && $condition2 ]] # Also works. 6 # Note that && operator not permitted within [ ... ] construct.

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

||

or (logical)

1 if [ $condition1 ] || [ $condition2 ] 2 # Same as: if [ $condition1 -o $condition2 ] 3 # Returns true if either condition1 or condition2 holds true... 4  5 if [[ $condition1 || $condition2 ]] # Also works. 6 # Note that || operator not permitted within [ ... ] construct.

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

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Example 8-3. Compound Condition Tests Using && and ||

1 #!/bin/bash 2  3 a=24 4 b=47 5  6 if [ "$a" -eq 24 ] && [ "$b" -eq 47 ] 7 then 8  echo "Test #1 succeeds." 9 else 10  echo "Test #1 fails." 11 fi 12  13 # ERROR: if [ "$a" -eq 24 && "$b" -eq 47 ] 14 #+ attempts to execute ' [ "$a" -eq 24 ' 15 #+ and fails to finding matching ']'. 16 # 17 # Note: if [[ $a -eq 24 && $b -eq 24 ]] works. 18 # The double-bracket if-test is more flexible 19 #+ than the single-bracket version. 20 # (The "&&" has a different meaning in line 17 than in line 6.) 21 # Thanks, Stephane Chazelas, for pointing this out. 22  23  24 if [ "$a" -eq 98 ] || [ "$b" -eq 47 ] 25 then 26  echo "Test #2 succeeds." 27 else 28  echo "Test #2 fails." 29 fi 30  31  32 # The -a and -o options provide 33 #+ an alternative compound condition test. 34 # Thanks to Patrick Callahan for pointing this out. 35  36  37 if [ "$a" -eq 24 -a "$b" -eq 47 ] 38 then 39  echo "Test #3 succeeds." 40 else 41  echo "Test #3 fails." 42 fi 43  44  45 if [ "$a" -eq 98 -o "$b" -eq 47 ] 46 then 47  echo "Test #4 succeeds." 48 else 49  echo "Test #4 fails." 50 fi 51  52  53 a=rhino 54 b=crocodile 55 if [ "$a" = rhino ] && [ "$b" = crocodile ] 56 then 57  echo "Test #5 succeeds." 58 else 59  echo "Test #5 fails." 60 fi 61  62 exit 0

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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.

1 let "t1 = ((5 + 3, 7 - 1, 15 - 4))" 2 echo "t1 = $t1" # t1 = 11 3  4 let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". 5 echo "t2 = $t2 a = $a" # t2 = 5 a = 9

The comma operator finds use mainly in for loops. See Example 10-12.