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Chapter 8. Operations and Related Topics8.1. Operatorsassignment  variable assignment
Initializing or changing the value of a variable  =
Allpurpose 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 924 and Example 927) and
formatting program output (see Example 2615 and
Example A6). It can even be used to generate
prime numbers, (see Example A16). Modulo turns
up surprisingly often in various numerical recipes. Example 81. 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` firstnumber secondnumber"
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 = $dividend"; echo
# Exercise :
# 
# Check commandline arguments to make sure they are integers,
#+ and exit the script with an appropriate error message if not.
exit 0  +=
"plusequal" (increment variable by a constant) let "var += 5" results in
var being incremented by
5.  =
"minusequal" (decrement variable by a constant)  *=
"timesequal" (multiply variable by a constant) let "var *= 4" results in var
being multiplied by 4.  /=
"slashequal" (divide variable by a constant)  %=
"modequal" (remainder of dividing variable by a constant) Arithmetic operators often occur in an
expr or let expression. Example 82. Using Arithmetic Operations #!/bin/bash
# Counting to 11 in 10 different ways.
n=1; echo n "$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 ))
# A simpler alternative to the method above.
# Thanks, David Lombard, for pointing this out.
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=$[ $n + 1 ]
# Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
# Thanks, Stephane Chazelas.
echo n "$n "
# Now for Cstyle increment operators.
# Thanks, Frank Wang, for pointing this out.
let "n++" # let "++n" also works.
echo n "$n "
(( n++ )) # (( ++n ) also works.
echo n "$n "
: $(( n++ )) # : $(( ++n )) also works.
echo n "$n "
: $[ n++ ] # : $[ ++n ]] also works
echo n "$n "
echo
exit 0
 Integer variables in Bash are actually signed
long (32bit) 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)
As of version 2.05b, Bash supports 64bit integers. 
 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  <<
bitwise left shift (multiplies by 2
for each shift position)  <<=
"leftshiftequal" let "var <<= 2" results in var
leftshifted 2 bits (multiplied by 4)  >>
bitwise right shift (divides by 2
for each shift position)  >>=
"rightshiftequal" (inverse of <<=)  &
bitwise and  &=
"bitwise andequal"  
bitwise OR  =
"bitwise ORequal"  ~
bitwise negate  !
bitwise NOT  ^
bitwise XOR  ^=
"bitwise XORequal"
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  $condition2 ]] # Also works.
# Note that  operator not permitted within [ ... ] construct.  Bash tests the exit
status of each statement linked with a logical
operator. 
Example 83. Compound Condition Tests Using && and  #!/bin/bash
a=24
b=47
if [ "$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 ']'.
#
# Note: if [[ $a eq 24 && $b eq 24 ]] works.
# The doublebracket iftest is more flexible
#+ than the singlebracket version.
# (The "&&" has a different meaning in line 17 than in line 6.)
# Thanks, Stephane Chazelas, for pointing this out.
if [ "$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 1012.
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