Advanced Bash-Scripting Guide
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Appendix B. Reference Cards

The following reference cards provide a useful summary of certain scripting concepts. The foregoing text treats these matters in more depth, as well as giving usage examples.

Table B-1. 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 command line parameters passed to script
${#@}	Number of command line parameters passed to script
$?	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

* Must be quoted, otherwise it defaults to "$@".

Table B-2. 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

* If within a double-bracket [[ ... ]] test construct, then no escape \ is needed.

Table B-3. TEST Operators: Files

Operator	Tests Whether	Operator	Tests Whether
-e	File exists	-s	File is not zero size
-f	File is a regular file
-d	File is a directory	-r	File has read permission
-h	File is a symbolic link	-w	File has write permission
-L	File is a symbolic link	-x	File has execute permission
-b	File is a block device
-c	File is a character device	-g	sgid flag set
-p	File is a pipe	-u	suid flag set
-S	File is a socket	-k	"sticky bit" set
-t	File is associated with a terminal

-N	File modified since it was last read	F1 -nt F2	File F1 is newer than F2 *
-O	You own the file	F1 -ot F2	File F1 is older than F2 *
-G	Group id of file same as yours	F1 -ef F2	Files F1 and F2 are hard links to the same file *

!	NOT (inverts sense of above tests)

* Binary operator (requires two operands).

Table B-4. Parameter Substitution and Expansion

Expression	Meaning
${var}	Value of var, same as $var

${var-DEFAULT}	If var not set, evaluate expression as $DEFAULT *
${var:-DEFAULT}	If var not set or is empty, evaluate expression as $DEFAULT *

${var=DEFAULT}	If var not set, evaluate expression as $DEFAULT *
${var:=DEFAULT}	If var not set, evaluate expression as $DEFAULT *

${var+OTHER}	If var set, evaluate expression as $OTHER, otherwise as null string
${var:+OTHER}	If var set, evaluate expression as $OTHER, otherwise as null string

${var?ERR_MSG}	If var not set, print $ERR_MSG *
${var:?ERR_MSG}	If var not set, print $ERR_MSG *

*${!varprefix}**	Matches all previously declared variables beginning with varprefix
${!varprefix@}	Matches all previously declared variables beginning with varprefix

* Of course if var is set, evaluate the expression as $var.

Table B-5. String Operations

Expression	Meaning
${#string}	Length of $string

${string:position}	Extract substring from $string at $position
${string:position:length}	Extract $length characters substring from $string at $position

${string#substring}	Strip shortest match of $substring from front of $string
${string##substring}	Strip longest match of $substring from front of $string
${string%substring}	Strip shortest match of $substring from back of $string
${string%%substring}	Strip longest match of $substring from back of $string

${string/substring/replacement}	Replace first match of $substring with $replacement
${string//substring/replacement}	Replace all matches of $substring with $replacement
${string/#substring/replacement}	If $substring matches front end of $string, substitute $replacement for $substring
${string/%substring/replacement}	If $substring matches back end of $string, substitute $replacement for $substring


expr match "$string" '$substring'	Length of matching $substring* at beginning of $string
expr "$string" : '$substring'	Length of matching $substring* at beginning of $string
expr index "$string" $substring	Numerical position in $string of first character in $substring that matches
expr substr $string $position $length	Extract $length characters from $string starting at $position
expr match "$string" '$$substring$'	Extract $substring* at beginning of $string
expr "$string" : '$$substring$'	Extract $substring* at beginning of $string
*expr match "$string" '.$$substring$'**	Extract $substring* at end of $string
*expr "$string" : '.$$substring$'**	Extract $substring* at end of $string

* Where $substring is a Regular Expression.

Table B-6. Miscellaneous Constructs

Expression	Interpretation

Brackets
if [ CONDITION ]	Test construct
if [[ CONDITION ]]	Extended test construct
Array[1]=element1	Array initialization
[a-z]	Range of characters within a Regular Expression

Curly Brackets
${variable}	Parameter substitution
${!variable}	Indirect variable reference
{ command1; command2; . . . commandN; }	Block of code
{string1,string2,string3,...}	Brace expansion
{a..z}	Extended brace expansion
{}	Text replacement, after find and xargs


Parentheses
( command1; command2 )	Command group executed within a subshell
Array=(element1 element2 element3)	Array initialization
result=$(COMMAND)	Command substitution, new style
>(COMMAND)	Process substitution
<(COMMAND)	Process substitution

Double Parentheses
(( var = 78 ))	Integer arithmetic
var=$(( 20 + 5 ))	Integer arithmetic, with variable assignment
(( var++ ))	C-style variable increment
(( var-- ))	C-style variable decrement
(( var0 = var1<98?9:21 ))	C-style trinary operation

Quoting
"$variable"	"Weak" quoting
'string'	'Strong' quoting

Back Quotes
result=`COMMAND`	Command substitution, classic style

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