Bash

The great GNU Bourne-Again Shell

Bash is the shell, or command language interpreter, that will appear in the GNU Operating System.

Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh).

Bash Links

• Bash GNU home
• Bash Introduction
• Bash Features
• FTP download
• The Bourne Again Shell

Useful Keyboard combinations in Bash

• ^a = start of line

Operators

Assignment Operators

=

All-purpose assignment operator, which works for both arithmetic and string assignments.Do not confuse the "=" assignment operator with the = test operator.

Arithmetic Operators

+

plus

-

minus

*

multiplication

/

division

**

exponentiation

%

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

+=

"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)

Logical Operators

&&

and (logical). if [ $condition1 ] && [ $condition2 ]

||

or (logical). if [ $condition1 ] || [ $condition2 ]

Dont confuse those with #Conditional executing.

Comparison Operators

Integers

-eq

is equal to. if [ "$a" -eq "$b" ]

-ne

is not equal to. if [ "$a" -ne "$b" ]

-gt

is greater than. if [ "$a" -gt "$b" ]

-ge

is greater than or equal to if. [ "$a" -ge "$b" ]

-lt

is less than. if [ "$a" -lt "$b" ]

-le

is less than or equal to. if [ "$a" -le "$b" ]

<

is less than. (within double parentheses) (("$a" < "$b"))

<=

is less than or equal to. (within double parentheses). (("$a" <= "$b"))

>

is greater than (within double parentheses). (("$a" > "$b"))

>=

is greater than or equal to (within double parentheses). (("$a" >= "$b"))

Strings

=

is equal to. if [ "$a" = "$b" ]

==

is equal to. if [ "$a" == "$b" ]

!=

is not equal to. if [ "$a" != "$b" ]

<

is less than, in ASCII alphabetical order

>

is greater than, in ASCII alphabetical order

-z

string is "null", that is, has zero length

-n

string is not "null".

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

Misc 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.

Conditional executing

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero. (exit status zero means succesful)

command1 || command2

command2 is executed if and only if command1 returns a non-zero exit status. (non-zero exit status means failed)

command1 && command2 || command3 

if command1 is executed successfully then shell will run command2 and if command1 is not successful then command3 is executed.

also see: Bash Scripts