List Of Especial Bash Parameter Used Inward Unix Or Linux Script

Meaning of bash parameter used inwards Unix script
Many of us purpose a bash script for doing housekeeping together with other materials but occasionally together with non much aware of special bash parameters. When I was novel to the bash compaction together with Linux together with looking for unopen to already written bash script I used to larn baffled alongside special bash symbols similar $@, $_, $1 etc. I did know that they are bash parameter together with has special pregnant but I don't bring all meanings of special bash parameter on top of my caput together with I e'er larn to Google together with search endlessly for those special bash parameter, sometimes I got together with institute pregnant of those special bash script parameters rapidly but sometimes I need to pass unopen to fourth dimension to larn what I am truly looking for, hence I idea to document meanings together with expansion of those special bash parameters or bash script parameters.

This bash script parameter tutorial is inwards continuation of my before UNIX tutorials find examples inwards UNIX, grep ascendancy examples inwards UNIX, together with UNIX networking commands tutorial if you lot haven’t read already you lot may discovery them interesting together with useful.

The listing which I am going to portion alongside you lot guys today is what I bring accumulated over the years. This listing of special bash parameters are yesteryear no agency consummate together with exclusively contains unopen to of the bash script parameters which I bring encountered , hence delight contribute whatever bash parameter which is non inwards this listing together with you lot institute useful.

What are special bash parameters?

Special bash parameters are those parameters which bash compaction treats peculiarly when encounters inwards a bash script inwards Linux or UNIX system. The of import call for of these bash script parameters is that they tin sack exclusively move referenced together with you lot tin sack non assign whatever value to them. Apart from special bash parameters I bring too listed downward the pregnant of special bash characters for quick reference e.g. "&"  ">" ">>“, these special bash characters are quite useful land working inwards bash compaction inwards UNIX or Linux environment. I know remembering these special bash parameters together with special characters are quite tricky peculiarly when you lot write bash script occasionally but at the same time, you lot need to sympathize it to read existing bash scripts written yesteryear somebody similar bash script experts. What I practise is I proceed this inwards a house which is quite handy together with merely search whenever I come upwards across whatever special bash parameters.

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Special bash parameters together with their meaning

Special bash parameter	Meaning
$!	$! bash script parameter is used to reference the procedure ID of the nigh of late executed ascendancy inwards background.
$$	$$ is used to reference the procedure ID of bash compaction itself
$#	$# is quite a special bash parameter together with it expands to a set out of positional parameters inwards decimal.
$0	$0 bash parameter is used to reference the advert of the compaction or compaction script. hence you lot tin sack purpose this if you lot desire to impress the advert of compaction script.
$-	$- (dollar hyphen) bash parameter is used to larn electrical flow selection flags specified during the invocation, yesteryear the laid built-in ascendancy or laid yesteryear the bash compaction itself. Though this bash parameter is rarely used.
$?	$0 is i of the nigh used bash parameters together with used to larn the larn out condition of the nigh of late executed ascendancy inwards the foreground. By using this you lot tin sack cheque whether your bash script is completed successfully or not.
$_	$_ (dollar underscore) is unopen to other special bash parameter together with used to reference the absolute file advert of the compaction or bash script which is beingness executed equally specified inwards the declaration list. This bash parameter is too used to concur the advert of post service file land checking emails.
$@	$@ (dollar at the rate) bash parameter is used to expand into positional parameters starting from one. When expansion occurs within double-quotes, every parameter expands into split upwards words.
$*	$* (dollar star) this is similar to $@ special bash parameter  exclusively departure is when expansion occurs alongside double quotes, it expands to a unmarried give-and-take alongside the value of each bash parameter separated yesteryear the outset grapheme of the IFS special surroundings variable.

Example of passing parameter to bash script inwards Unix:

Many times nosotros require a passing parameter to bash script. Now let’s see an representative of bash script on which nosotros volition move yesteryear bash parameter from the ascendancy prompt together with volition access them within the bash script. This representative volition assistance you lot to sympathize the purpose of special bash parameters much better.

$ truthful cat bashparameterexample.sh

#!/bin/bash

if [ $# -lt 2 ]   -- $# is used for set out of arguments

then

  echo "Usage: $0 arg1 arg2"

  exit

fi

echo -e  "\$1=$1" -- $1 is used to access outset parameter

echo -e "\$2=$2"  -- $2 is used to access instant parameter

Now let's invoke the bash script without parameter, it volition impress the “usage “statement because $# is less than 2, directly you lot see how useful “$#” is for writing script J

Javin@stocktrading  

$ ./bashparameterexample.sh      

Usage: ./bashparameterexample.sh arg1 arg2

Now lets move yesteryear 2 parameters to the script together with access them via $1 and

 $2 its truly fun writing bash script i time you lot are familiar alongside bash parameters.

Javin@stocktrading  

$ ./bashparameterexample.sh  23 44

$1=23

$2=44

Difference betwixt $* together with $@ bash parameters

$* together with $@ both are used to access total listing of positional parameters supplied during invocation of bash script together with you lot tin sack iterate through them using bash for loop. $* together with $@ are special bash parameters together with if used without double quotes hence both volition acquit identical i.e. they volition render the listing of positional parameter starting from the outset parameter $1 together with each of those volition move separated yesteryear space.

For example, if you're render five parameters during invocation of bash script $@ together with $* volition expand into $1 $2 $3 $4 $5

But if you lot double quotes hence they volition acquit differently, inwards the instance of $* private parameters volition move separated yesteryear first char of IFS characters land inwards instance of $@ private parameters volition appear inwards quotes together with separated yesteryear space.

Special bash characters together with their meaning

We bring merely seen special bash script parameters together with their pregnant together with directly nosotros volition unopen to special bash characters together with what they hateful when used within bash script or inwards bash compaction ascendancy prompt. We volition too see special bash characters called redirection characters which are used to redirect input, output etc

Special bash character	Meaning
#	# is used to comment a unmarried describe of piece of job inwards bash script
$$	$$ is used to reference procedure id of whatever ascendancy or bash script
$0	$0 is used to larn the advert of the ascendancy inwards a bash script.
$name	$name volition impress the value of variable “name” defined inwards the script.
$n	$n volition impress the value of nth declaration provided to bash script (n ranges from 0 to 9) e.g. $1 volition impress outset argument.
>	> is used to redirect output
>>	>> tin sack move used to Append to file
<	< volition redirect input
[ ]	[] are for matching whatever characters enclosed
( )	Execute inwards subshell
` `	Another powerful special bash grapheme which is used to substitute output of enclosed command
" "	Partial quote (allows variable together with ascendancy expansion)
' '	Full quote (no expansion)
\	Quote next character
|	Pipe output of i ascendancy to unopen to other nigh useful together with immensely ability bash character
&	& is used to run whatever procedure inwards the background.
;	;(semi colon ) is used to split upwards commands on same line
*	* is used to jibe whatever character(s) inwards filename
?	? is for matching unmarried grapheme inwards filename

Knowledge of special bash parameters together with special bash characters volition brand you lot to a greater extent than comfortable land reading together with sympathize already written bash scripts. It is this cryptic bash parameter which makes the script to a greater extent than complex or reading but at the same fourth dimension tin sack move immensely useful if know how to purpose them. These special bash parameters tin sack move used inwards whatever bash script.

Further Learning
Linux Command Line Basics
UNIX Command Tutorials together with Tips for Beginners

File together with Directory Permissions inwards UNIX together with Linux

How to discovery IP address from hostname inwards UNIX

List of Special Bash Parameters inwards UNIX

10 Tips to move fast inwards UNIX together with Linux

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