List Of Especial Bash Parameter Used Inward Unix Or Linux Script

Meaning of bash parameter used inward Unix script
Many of us role a bash script for doing housekeeping in addition to other materials but occasionally in addition to non much aware of special bash parameters. When I was novel to the bash vanquish in addition to Linux in addition to looking for about already written bash script I used to larn baffled amongst special bash symbols similar $@, $_, $1 etc. I did know that they are bash parameter in addition to has special pregnant but I don't convey all meanings of special bash parameter on top of my caput in addition to I e'er become to Google in addition to search endlessly for those special bash parameter, sometimes I got in addition to constitute pregnant of those special bash script parameters speedily but sometimes I ask to pass about fourth dimension to larn what I am genuinely looking for, thus I idea to document meanings in addition to expansion of those special bash parameters or bash script parameters.

This bash script parameter tutorial is inward continuation of my before UNIX tutorials find examples inward UNIX, grep ascendance examples inward UNIX, in addition to UNIX networking commands tutorial if you lot haven’t read already you lot may discovery them interesting in addition to useful.

The listing which I am going to percentage amongst you lot guys today is what I convey accumulated over the years. This listing of special bash parameters are past times no way consummate in addition to solely contains about of the bash script parameters which I convey encountered , thus delight contribute whatever bash parameter which is non inward this listing in addition to you lot constitute useful.

What are special bash parameters?

Special bash parameters are those parameters which bash vanquish treats specially when encounters inward a bash script inward Linux or UNIX system. The of import indicate of these bash script parameters is that they tin flaming solely live on referenced in addition to you lot tin flaming non assign whatever value to them. Apart from special bash parameters I convey likewise listed downward the pregnant of special bash characters for quick reference e.g. "&"  ">" ">>“, these special bash characters are quite useful spell working inward bash vanquish inward UNIX or Linux environment. I know remembering these special bash parameters in addition to special characters are quite tricky specially when you lot write bash script occasionally but at the same time, you lot ask to empathise it to read existing bash scripts written past times person similar bash script experts. What I practise is I give-up the ghost along this inward a house which is quite handy in addition to simply search whenever I come upward across whatever special bash parameters.

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Special bash parameters in addition to their meaning

Special bash parameter	Meaning
$!	$! bash script parameter is used to reference the procedure ID of the virtually of late executed ascendance inward background.
$$	$$ is used to reference the procedure ID of bash vanquish itself
$#	$# is quite a special bash parameter in addition to it expands to a pose out of positional parameters inward decimal.
$0	$0 bash parameter is used to reference the parent of the vanquish or vanquish script. thus you lot tin flaming role this if you lot desire to impress the parent of vanquish script.
$-	$- (dollar hyphen) bash parameter is used to larn electrical flow pick flags specified during the invocation, past times the laid built-in ascendance or laid past times the bash vanquish itself. Though this bash parameter is rarely used.
$?	$0 is 1 of the virtually used bash parameters in addition to used to larn the travel out condition of the virtually of late executed ascendance inward the foreground. By using this you lot tin flaming banking concern check whether your bash script is completed successfully or not.
$_	$_ (dollar underscore) is about other special bash parameter in addition to used to reference the absolute file parent of the vanquish or bash script which is existence executed every bit specified inward the declaration list. This bash parameter is likewise used to concur the parent of post service file spell 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 upward words.
$*	$* (dollar star) this is similar to $@ special bash parameter  solely divergence is when expansion occurs amongst double quotes, it expands to a unmarried give-and-take amongst the value of each bash parameter separated past times the offset grapheme of the IFS special surroundings variable.

Example of passing parameter to bash script inward Unix:

Many times nosotros require a passing parameter to bash script. Now let’s reckon an instance of bash script on which nosotros volition transcend bash parameter from the ascendance prompt in addition to volition access them within the bash script. This instance volition assistance you lot to empathise the role of special bash parameters much better.

$ truthful cat bashparameterexample.sh

#!/bin/bash

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

then

  echo "Usage: $0 arg1 arg2"

  exit

fi

echo -e  "\$1=$1" -- $1 is used to access offset 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 $# is less than 2, straight off you lot reckon how useful “$#” is for writing script J

Javin@stocktrading  

$ ./bashparameterexample.sh      

Usage: ./bashparameterexample.sh arg1 arg2

Now lets transcend 2 parameters to the script in addition to access them via $1 and

 $2 its genuinely fun writing bash script 1 time you lot are familiar amongst bash parameters.

Javin@stocktrading  

$ ./bashparameterexample.sh  23 44

$1=23

$2=44

Difference betwixt $* in addition to $@ bash parameters

$* in addition to $@ both are used to access total listing of positional parameters supplied during invocation of bash script in addition to you lot tin flaming iterate through them using bash for loop. $* in addition to $@ are special bash parameters in addition to if used without double quotes in addition to thus both volition demeanor identical i.e. they volition furnish the listing of positional parameter starting from the offset parameter $1 in addition to each of those volition live on separated past times space.

For example, if you're furnish v parameters during invocation of bash script $@ in addition to $* volition expand into $1 $2 $3 $4 $5

But if you lot double quotes in addition to thus they volition demeanor differently, inward the instance of $* private parameters volition live on separated past times first char of IFS characters spell inward instance of $@ private parameters volition appear inward quotes in addition to separated past times space.

Special bash characters in addition to their meaning

We convey simply seen special bash script parameters in addition to their pregnant in addition to straight off nosotros volition about special bash characters in addition to what they hateful when used within bash script or inward bash vanquish ascendance prompt. We volition likewise reckon special bash characters called redirection characters which are used to redirect input, output etc

Special bash character	Meaning
#	# is used to comment a unmarried job inward bash script
$$	$$ is used to reference procedure id of whatever ascendance or bash script
$0	$0 is used to larn the parent of the ascendance inward a bash script.
$name	$name volition impress the value of variable “name” defined inward 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 offset argument.
>	> is used to redirect output
>>	>> tin flaming live on used to Append to file
<	< volition redirect input
[ ]	[] are for matching whatever characters enclosed
( )	Execute inward subshell
` `	Another powerful special bash grapheme which is used to substitute output of enclosed command
" "	Partial quote (allows variable in addition to ascendance expansion)
' '	Full quote (no expansion)
\	Quote next character
|	Pipe output of 1 ascendance to about other virtually useful in addition to immensely ability bash character
&	& is used to run whatever procedure inward the background.
;	;(semi colon ) is used to split upward commands on same line
*	* is used to gibe whatever character(s) inward filename
?	? is for matching unmarried grapheme inward filename

Knowledge of special bash parameters in addition to special bash characters volition brand you lot to a greater extent than comfortable spell reading in addition to empathise 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 flaming live on immensely useful if know how to role them. These special bash parameters tin flaming live on used inward whatever bash script.

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