Linux Bash internal variable usage example

$ @

"$ @" Returns all command line parameters as an array. Unlike "$ *", it is returned as a string.
"$ @" Can traverse all elements through loops, as shown in the following script:

#! /Bin/bash
For var in "$ *"; do
Echo $ var
Done
Because $ * only returns a parameter as a string, echo is called only once:

~> $./Testscript. sh firstarg secondarg thirdarg
Firstarg secondarg thirdarg
When using $:

#! /Bin/bash
For var in "$ @"; do
Echo $ var
Done
Return all parameters in an array. You can access each parameter separately:

~> $./Testscript. sh firstarg secondarg thirdarg
Firstarg
Secondarg
Thirdarg
$ #

Obtain the number of command line parameters, and type:

#! /Bin/bash
Echo "$ #"
If the script contains three parameters, the output is as follows:

~> $./Testscript. sh firstarg secondarg thirdarg
3
$!

Returns the ID of the process executed by the previous program:

~> $ Ls &
Testfile1 testfile2
[1] + Done ls
~> $ Echo $!
21715
$

If the pid of the current process is executed under the bash command line, it is equivalent to the pid of the bash process:

~> $ Echo $
13246
$ *

Returns all command line parameters with a single string.
Testscript. sh:

#! /Bin/bash
Echo "$ *"
Run the script with several parameters:

./Testscript. sh firstarg secondarg thirdarg
Output:

Firstarg secondarg thirdarg
$?

Returns the exit status of the upstream function or command execution. 0 indicates that the execution is successful. Otherwise, the execution fails:

~> $ Ls *. blah; echo $?
Ls: cannot access *. blah: No such file or directory
2
~> $ Ls; echo $?
Testfile1 testfile2
0
$1 $2 $3, etc.

The location parameter that is passed to the script or a function from the command line:

#! /Bin/bash
# $ N is the n 'th positional parameter
Echo "$1"
Echo "$2"
Echo "$3"
The output is as follows:
~> $./Testscript. sh firstarg secondarg thirdarg

Firstarg
Secondarg
Thirdarg
If the number of positional parameters is greater than 9, braces must be used:

# "Set --" sets positional parameters
Set -- 1 2 3 4 5 6 7 8 nine ten eleven twelfe
Echo $10 # outputs 1
Echo $ {10} # outputs ten
$ FUNCNAME

Get the name of the current function:

My_function ()
{
Echo "This function is $ FUNCNAME" # This will output "This function is my_function"
}
If this variable is printed out of the function:

My_function
 
Echo "This function is $ FUNCNAME" # This will output "This function is"
$ HOME

User's home directory

~> $ Echo $ HOME
/Home/user
$ IFS

This variable contains the internal field separator used to split the string by bash in a loop. By default, it is a blank character \ n (newline), \ t (tab), or space. If you change it, you can use different separators to split strings:

IFS = ","
INPUTSTR = "a, B, c, d"
For field in $ {INPUTSTR}; do
Echo $ field
Done
Output:
A
B
C
D

$ PWD

Output current working directory

~> $ Echo $ PWD
/Home/user
~> $ Cd directory
Directory> $ echo $ PWD
/Home/user/directory
$ HOSTNAME

Host Name assigned at system startup

~> $ Echo $ HOSTNAME
Mybox.mydomain.com
$ LINENO

The current row number of the output script. It may be used in script debugging.

#! /Bin/bash
# This is line 2
Echo something # this is line 3
Echo $ LINENO # Will output 4