1. Position variable
(1) $ ==> Take file name
[email protected] ~]# cat test.sh
Echo
[Email protected] ~]# sh test.sh
test.sh
[Email protected] ~]# sh ~/test.sh
/root/test.sh
Description: When executing a script, the script name is preceded by a path to the output path. The path is to be output if there is a path ahead. You can also use commands dirname and basename to take a value.
(2) $ #和 $n
$# ==> Number of parameters
Note: "" can only be counted as a variable, here set three variables first.
[[Email protected] ~]# set--"I am" ugly oldboy
[Email protected] ~]# echo $#
3
[Email protected] ~]# echo $
I am
[Email protected] ~]# Echo
Ugly
[Email protected] ~]# echo $3br/>oldboy
(3) The difference between $* and [email protected]
[email protected]~]# echo $</span>
I am Ugly Oldboy
[email protected] ~]# echo [email protected]
I am Ugly Oldboy
[[email protected] ~]# for i in "$";d o echo $i;d one
I am Ugly Oldboy ==> The $* has only one parameter
[ [email protected]~]# for I in " [email protected]";d o echo $i;d one
I am
Ugly
Oldboy ==> [email protected]There are four parameters
2. Process state variables
(1) $? 0 for success, not 0 for failure
[ [email protected]~]# Date
May 01, 2018 Tuesday 18:58:42 CST
[ [email protected]~]# echo $?
0
[ [email protected]~]# RM/
RM: Cannot delete "/": is a directory
[ [email protected]~]# echo $?
1
In the enterprise scenario, the use of the $? Return value is as follows:
① to determine whether a program, such as a command, script, or function is successfully executed
② call Exit N in the script, will return this number to $?.
③ in the function, the number is returned by return N to the value of the function return to $?
(2) $$ get process number
Validation method ①:[ [email protected]~]# Cat testpid.sh
Echo $$
[ [email protected]~]# SH testpid.sh &
[3] 3357
[ [email protected]~]# 3357
[3]+ done sh testpid.sh
② (recommended): [[ Email protected] ~]# su-thzzc
[[email protected] ~]$ echo $$
2544
[[email protected] ~]$ kill-9 2544
[[email protected] ~]#
(3) $ The last argument of the last command (more than arguments, the same as the command, should be a word)
[[ Email protected] ~]# echo && echo $
Echo
[[email protected] ~]# echo Kobe && echo $
Kobe
Kobe
(4) $! Gets the process number of the last script
[email protected] ~]# cat testpid.sh
Sleep 100
[email protected] ~]# ps-ef |grep testpid
Root 6557 6311 0 15:48 pts/0 00:00:00 grep testpid
[[email protected] ~]# sh testpid.sh &
[1] 6558
[email protected] ~]# echo $!
6558
Summary: Position variable $ $n $# $* [email protected] 5 and process variables $$ $! $? $ 4, all written down will be of great help to shell programming.
Shell programming in the 4th Chapter Shell variable Advanced (upper)
