Adv Bash - 1

Special Characters

What makes a character special? If it has a meaning beyond its literal meaning, a meta-meaning, then we refer to it as a special character. Along with commands and keywords, special characters are building blocks of Bash scripts.

Special Shell Variables

Variable	Meaning
$0	Filename of script
$1	Positional parameter #1
$2 - $9	Positional parameters #2 - #9
${10}	Positional parameter #10
$#	Number of positional parameters
”$*”	All the positional parameters (as a single word) *
”$@”	All the positional parameters (as separate strings)
${#*}	Number of positional parameters
${#@}	Number of positional parameters
$?	Return value
$$	Process ID (PID) of script
$-	Flags passed to script (using set)
$_	Last argument of previous command
$!	Process ID (PID) of last job run in background

Operator Precedence

In a script, operations execute in order of precedence: the higher precedence operations execute before the lower precedence ones.

Operator	Meaning	Comments
	HIGHEST PRECEDENCE
var++ var–	post-increment, post-decrement	C-style operators
++var –var	pre-increment, pre-decrement
! ~	negation	logical / bitwise, inverts sense of following operator
**	exponentiation	arithmetic operation
* / %	multiplication, division, modulo	arithmetic operation
+ -	addition, subtraction	arithmetic operation
<< >>	left, right	shift bitwise
-z -n	unary comparison	string is/is-not null
-e -f -t -x, etc.	unary comparison	file-test
< -lt > -gt <= -le >= -ge	compound comparison	string and integer
-nt -ot -ef	compound comparison	file-test
== -eq != -ne	equality / inequality	test operators, string and integer
&	AND	bitwise
^	XOR	exclusive OR, bitwise
|	OR	bitwise
&& -a	AND	logical, compound comparison
| -o	OR	logical, compound comparison
?:	trinary operator	C-style
=	assignment	(do not confuse with equality test)
*= /= %= += -= <<= >>= &=	combination assignment	times-equal, divide-equal,
,	comma	links a sequence of operations
		LOWEST PRECEDENCE

TEST Operators: Binary Comparison

Operator	Meaning	Operator	Meaning
Arithmetic Comparison		String Comparison
-eq	Equal to	=	Equal to
		==	Equal to
-ne	Not equal to	!=	Not equal to
-lt	Less than < Less than (ASCII) *
-le	Less than or equal to
-gt	Greater than	>	Greater than (ASCII) *
-ge	Greater than or equal to
		-z	String is empty
		-n	String is not empty
Arithmetic Comparison	within double parentheses (( … ))
>	Greater than
>=	Greater than or equal to
<	Less than
<=	Less than or equal to