|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.
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.
modulo, or mod (returns the remainder of an integer division)
"plus-equal" (increment variable by a constant)
`expr $var+=5` results in var being incremented by 5.
"minus-equal" (decrement variable by a constant)
"times-equal" (multiply variable by a constant)
`expr $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)
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)
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 <<=)
less than or equal to
greater than or equal to
equal to (test)
not equal to
if [ $condition1 ] && [ $condition2 ] # if both condition1 and condition2 hold true...
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 octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # is evaluated as BASE#NUMBER (this option is of limited usefulness because of range restrictions).
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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