Multiple uses of the ‘$’ symbol in Shell scripts

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Multiple uses of the ‘$’ symbol in Shell scripts

Various uses of the ‘$’ symbol in Shell scripts

In general, the following are the most commonly used items at work:

The $sign	instructions
$0	The Shell command itself
$1 to$9	Represents the number of arguments of the Shell
$?	Displays the execution of the last command
$#	The number of arguments passed to the script
$$	ID of the process in which the script is running
$*	Displays all arguments passed to the script as a single string
$!	The ID of the last process to run in the background
$-	Displays the current options used by the Shell

Today we will pass the above options and make further operational cases;

1.1 Reference Variables

When referencing variables, use the $sign directly to reference them, and include loop variables;

[root@localhost ~]# x=1024
[root@localhost ~]# echo $x
1024
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String enclosed by double quotation marks supports variable interpolation.

[root@localhost ~]# x=1024
[root@localhost ~]# echo "x = $x"
x = 1024
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Use ${} for word boundaries.

[root@localhost ~]# x=1024
[root@localhost ~]# echo "x = ${x}xy"
x = 1024xy
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Use ${#} to get the variable string length.

[root@localhost etc]# s=helloworld
[root@localhost etc]# echo "s.length = ${#s}"
s.length = 10
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1.2 Referencing script or function parameters

Based on the way the script is referenced, 1 represents the Shell script file name, n starts at 2 for the NTH argument, and the second argument is $2.

[root@localhost ~]# echo 'echo $1 $2 $3' > ping.sh
[root@localhost ~]# cat ping.sh
echo $1 $2 $3
[root@localhost ~]# sh ping.sh 1 2 3
1 2 3
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Strings enclosed in single quotes are not interpolated and use $# to get the number of script or function arguments;

[root@localhost ~]# echo 'echo $#' > ping.sh
[root@localhost ~]# sh ping.sh 1 2 3
3
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1.3 Return value of the previous command

Using the $? The return value of the previous command.

0: indicates that there are no errors. Other values: indicates that there are errors.

[root@localhost ~]# true 1024
[root@localhost ~]# echo $?
0
[root@localhost ~]# false 2048
[root@localhost ~]# echo $?
1
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1.4 Executing and obtaining the command output

Execute with $() and get the command output assigned to a variable, equal to double quotes.

[root@localhost ~]# echo 'date' Sunday, June 05, 2016 12:39:08 CST [root@localhost ~]# echo $(Date) Sunday, June 05, 2016 12:39:34 CSTCopy the code

1.5 Expression evaluation

[root@localhost ~]# echo $[1024 + 2048]
3072
[root@localhost ~]# expr 1024 + 2048
3072
[root@localhost ~]# a=1024
[root@localhost ~]# b=2048
[root@localhost ~]# echo $[ a + b ]
3072
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1.6 Obtaining the CURRENT Process ID

Use $$to get the ID of the current process.

[root@localhost ~]# echo $$
55580
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1.7 ID of the last process running in the background

Using the $! To get the ID of the last process running in the background.

Use & at the end of the command to create a background process.

[root@localhost ~]# tail -f /root/ping.sh & [2] 55848 [root@localhost ~]# echo $! 55848 [root@localhost ~]# kill $! [root@localhost ~]# echo $! 55848 [2]+ Tail -f /root/ping.shCopy the code

1.8 Obtaining Shell options

Use $- to get the current Shell option.

[root@localhost ~]# echo $-
himBH
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Recommended reading

Super hardcore! 11 very useful Python and Shell scripts to use examples!

7 very useful Shell take to use script examples!

Shell programming – condition test | base paper

Shell programming – control structure | foundation

Shell programming | script parameters and the interaction and common problems

In this paper, to the end.

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