Appendix B. Reference Cards

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2023-12-01

The following reference cards provide a usefulsummaryof certain scripting concepts.The foregoing text treats these matters in more depth and givesusage examples.

Table B-1. Special Shell Variables

Variable	Meaning
`$0`	Name 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 toscript
`${#@}`	Number of command line parameters passed toscript
`$?`	Return value
`$$`	Process ID (PID) of script
`$-`	Flags passed to script (usingset)
`$_`	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 adouble-bracket[[ ... ]] test construct,then no escape\ isneeded.

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 regularfile
`-d`	File is a directory	`-r`	File has readpermission
`-h`	File is a symbolic link	`-w`	File has writepermission
`-L`	File is a symbolic link	`-x`	File has executepermission
`-b`	File is a block device
`-c`	File is a character device	`-g`	sgidflag set
`-p`	File is a pipe	`-u`	suidflag set
`-S`	File is a socket	`-k`	"sticky bit"set
`-t`	File is associated with aterminal

`-N`	File modified since it was last read	`F1 -nt F2`	File F1 is newerthan F2 *
`-O`	You own the file	`F1 -ot F2`	File F1 is olderthan F2 *
`-G`	Group idof file same asyours	`F1 -ef F2`	Files F1 and F2 are hard linksto the same file *

`!`	"NOT"(reverses sense of abovetests)

*Binaryoperator(requires two operands).

Table B-4. Parameter Substitution and Expansion

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

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

`${var=DEFAULT}`	If `var`not set, evaluate expressionas `$DEFAULT`*
`${var:=DEFAULT}`	If `var`not set, evaluate expressionas `$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 varisset, 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 allmatches of`$substring`with`$replacement`
`${string/#substring/replacement}`	If `$substring`matches frontend of`$string`, substitute`$replacement`for`$substring`
`${string/%substring/replacement}`	If `$substring`matches backend of`$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 `$length`charactersfrom `$string`starting at`$position`
`expr match "$string"'$$substring$'`	Extract `$substring`* atbeginning of `$string`
`expr "$string" :'$$substring$'`	Extract `$substring`* atbeginning of `$string`
`expr match "$string"'.*$$substring$'`	Extract `$substring`* atend of `$string`
`expr "$string" :'.*$$substring$'`	Extract `$substring`* atend of `$string`

*Where $substringis aregular 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 }`	Block of code
`{string1,string2,string3,...}`	Brace expansion


Parentheses
`( command1; command2 )`	Command group executed within a subshell
`Array=(element1 element2 element3)`	Array initialization
`result=$(COMMAND)`	Execute command in subshell and assign result tovariable
`>(COMMAND)`	Process substitution
`<(COMMAND)`	Process substitution

Double Parentheses
`(( var = 78 ))`	Integer arithmetic
`var=$(( 20 + 5 ))`	Integer arithmetic, with variable assignment

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

Back Quotes
result=`COMMAND`	Execute command in subshell and assign result to variable