The value and usage of Dollar $ in UNIX/LINUX and shell scripting

Dollar($) can be used to get the value of user-defined variable and also system-defined variables like HOME,LOGNAME,PATH,PS1,IFS,SHELL etc..
[root@localhost ~]# x=18
[root@localhost ~]# echo $x
[root@localhost ~]# echo $PATH
[root@localhost ~]#

Some times when you login to a particular user(other than root -> since root always has prompt #)of  UNIX/LINUX server the prompt appears as Dollar($) it’s because of the primary Prompt String(PS1) has been set a like below  .If you  want you can change it as your customized prompt.
[root@localhost ~]# echo $PS1
[\u@\h \W]\$
[root@localhost ~]# su - bharath
[bharath@localhost ~]$

$0 gives the name of the shell or shell script. "$0" gives the running script name with absolute path. If you want only the  script name you have to use  basename $0 which gives the name of the script alone.
[root@localhost ios]# echo $0
[root@localhost ios]# echo $SHELL


1.       These are so-called positional parameters represents a shell-scripts command-line arguments. Individual arguments are named by integer numbers.
2.       We also use $1,$2…. As a fields in order to extract and denote particular fields(columns) using awk utility so that we can do further action on extracted fields as below.

[root@localhost ios]# ls -lrt
total 12
-rwxrwxrwx. 1 root root  45 Dec  2 12:06 sample_script
-rwxrwxrwx. 1 root root 154 Dec  3 09:46 positional1
-rwxrwxrwx. 1 root root 152 Dec  3 09:47 positional
[root@localhost ios]# ls -lrt | awk '{print $1,$9}'
-rwxrwxrwx. sample_script
-rwxrwxrwx. positional1
-rwxrwxrwx. positional
[root@localhost ios]# ls -lrt | awk '{print $1,$9}' | awk '{print $2}' | xargs gzip
[root@localhost ios]# ls -lrt
total 12
-rwxrwxrwx. 1 root root  79 Dec  2 12:06 sample_script.gz
-rwxrwxrwx. 1 root root 115 Dec  3 09:46 positional1.gz
-rwxrwxrwx. 1 root root 115 Dec  3 09:47 positional.gz

It gives the number of positional parameters in decimal i.e. Total number of the passed to the shell script.
[root@localhost ios]# cat sample_script                                  Script execution:
#! /bin/bash                                                                                   [root@localhost ios]# sample_script a b c d e
echo "the value of \$# is  " $#                                                         the value of $# is   5
[root@localhost ios]#                                                                      [root@localhost ios]#

Scenario6->$@ and $*:

Both $@ and $* represents all the command line arguments(positional parameters) at once if and only if they used in with-out  double quotes i.e. both $* and $@ gives the positional parameters, starting from one if they used in without double quotes.
$@ and $* with-out double quotes:

$@ with-in Double quotes represents each parameter expands to a separate word. i.e. "$@" is equivalent to "$1" "$2"….. , $* with-in double quotes represents each parameter expands to a single string i.e. "$*" is equivalent to "$1$2….."

$@ and $* with-in double quotes:

Scenario7->$$ :
$$ gives the process ID of the current shell.
[root@localhost ios]# echo $$   --à bash shell
[root@localhost ios]# ps -eaf | grep  "\<2680\>" | egrep -v "(ps|grep)"
root      2680  2676  0 09:36 pts/1    00:00:00 -bash
[root@localhost ios]#

[root@localhost ios]# sh  -à changing the shell from bash to bourne i.e. bash to sh
sh-4.1# echo $$
sh-4.1# ps -eaf | grep "\<3135\>" | egrep -v "(ps|grep)"
root      3135  2680  0 10:33 pts/1    00:00:00 sh

Scenario8->$? : 
$? Gives the exit status of the most recently executed command or shell script. If the value of $?=0 means the command executed successfully if it is other than 0 means it indicates there is an error. $? Variable often used in shell scripts.
[root@localhost ios]# tar -cvf  ./backup.tar  *.gz
[root@localhost ios]# echo $?
[root@localhost ios]# ls -lrt
total 24
-rwxrwxrwx. 1 root root    79 Dec  2 12:06 sample_script.gz
-rwxrwxrwx. 1 root root   115 Dec  3 09:46 positional1.gz
-rwxrwxrwx. 1 root root   115 Dec  3 09:47 positional.gz
-rw-r--r--. 1 root root 10240 Dec  3 10:42 backup.tar

Scenario9->$! : 
$! Gives the process ID of the most recently executed command in background.
[root@localhost ios]# (sleep 50 ; who ; cal)&
[1] 3202
[root@localhost ios]# jobs
[1]+  Running                 ( sleep 50; who; cal ) &
[root@localhost ios]# echo $!

Scenario 10->$-(hyphen):
$- is a special variable used to represent the currently enabled shell options as below. Each options short option letter appears in the string if that option is enabled.
[root@localhost ~]# echo $-
-h => Remember the location of commands as they are looked up.(hash command enabled).
-i => shell is interactive shell.
-m => Job control is enabled.
-B => the shell will perform Brace expansion
-H => Enable ! style history substitution. This flag is on

Scenario 11-> $_(underscore):  
$_  gives the Last argument of the previous foreground executed command. If no argument given $_ displays the command itself.

Scenario 12->$< :
$<  is used for reading standard input. $< is a special c-shell variable used to read standard input into a variable.

Scenario 13-> $ in Regex :
$ in Regular expression is one of the Anchor character(another one is ^) used to match the end of line. Whereas ^ is used to match beginning of a line.

Scenario 14->$ in Arithmetic expansion:
$ used to do the arithmetic calculations in the command-line as well as in scripting.

[root@localhost ~]# echo $((x=12*60+30))
[root@localhost ~]#

[root@localhost ~]# echo $(( 3<4 || 5>6))
[root@localhost ~]# echo $(( 3<4 && 5<6))
[root@localhost ~]# echo $(( 3<4 && 5>6))
[root@localhost ~]#


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