1. Description of position variables

$0    Gets the file name of the currently executed script
$n	Gets the second instance of the currently executing script n Two parameters, n If it is greater than 10, use braces to pass parameters
$#	Gets the total number of all subsequent parameters in the current script
$*	Gets the parameters of all parameters passed in the current script
$@	Gets the parameters of all parameters passed in the current script

There is no difference between $@ and $@ without double quotation marks. When double quotation marks are added, $"" treats all parameters as a single string, and $"@" treats all strings as different independent strings

2. $0, $#, $n joint presentation

• cat test.sh

if [ $# -ne 2 ]	#If the passed in parameter is not 2
  then
     echo "/bin/sh $0 arg1 arg2"	#Prompt user for script path and usage
     exit 1	#Exit and return to 1
 fi
   echo $1 $2	#If two parameters are met, execute the command

• sh test.sh /bin/sh test.sh arg1 arg2. Prompt the user for usage without parameters
• sh test.sh ni wo ni wo correct execution results

2, shell state variable

1. Special state variables

2.$? Detailed use

1. Among the execution results of different commands, $? The return values of are different, but the common states are 0 and non-0. 0 indicates success and non-0 indicates failure. You can determine whether the program is executed successfully by obtaining the return value.
2. When backing up data, after executing key commands, obtain the return value to determine whether the command is successful and whether the backup data is completed

• In the enterprise scenario, $? The usage of is as follows
  • Judge whether the program such as command, script or function is executed successfully
  • If you execute "exit number" in the script, this number will be returned to $? variable
  • If it is a re function, pass the "return number" to $? S

3.$? Script case explanation

• sed -n '63,73p' /etc/init.d/rpcbind

stop() {
        echo -n $"Stopping $prog: "
        killproc $prog
        RETVAL=$?	#Will $? The value of is assigned to the variable RETVAL
        echo
        [ $RETVAL -eq 0 ] && {	#Judge that when the return value is 0, perform the following operations
                rm -f /var/lock/subsys/$prog
                rm -f /var/run/rpcbind*
        }
        return $RETVAL

If the return value is not 0, the command is not executed, and the return value is passed to the stop script

3, shell special variables

1. Grammar and function

Colons are not required. If you omit colons, you can add colons only to unassigned variables to include empty variables

2. Demonstration case

• cat /server/scripts/t2.sh

echo 'echo ${var:-ttt}-->' ${var:-ttt}
echo 'echo $var-->' $var
echo 'echo ${var:=ttt}-->' ${var:=ttt}
echo 'echo $var-->' $var
echo 'echo ${var:+MMM}-->' ${var:+MMM}
echo 'echo $var-->' $var
echo 'unset var and echo ${var:?the error}-->'
unset var
echo ${var:?the error} 

• sh /server/scripts/t2.sh

echo ${var:-ttt}--> ttt
echo $var-->
#If the variable is not assigned, use ttt instead of output to check that the variable is still not assigned
echo ${var:=ttt}--> ttt
echo $var--> ttt
#If the variable is not assigned, use ttt instead of output to check that the variable has been assigned
echo ${var:+MMM}--> MMM
echo $var--> ttt
#If the variable has been assigned a value, use MMM instead of output to check that the variable is still ttt
unset var and echo ${var:?the error}-->
/server/scripts/t2.sh: line 9: var: the error
#If the variable is not assigned, the defined alarm information is output

3. Business purpose

In enterprises, the processing of directory paths can be done by assigning values if the above variables do not exist, so as to prevent exceptions caused by the absence of directory paths, especially the deletion of variables. This method is very useful. Otherwise, when the deleted variables do not exist, unknown dangers may be caused find ${path:-/tmp} -name "*.tar.gz" -type f -mtime +7|xargs rm -f

With the above command, when the path variable does not exist, use / tmp path instead