SHELL variables for the collation of notes

Types of variables:

1. Custom variables

Define variables: Variable name = variable value variable name must start with a letter or underscore, case sensitive url1=test.3glong.com
Reference variable: $ variable name or ${variable name}
View variables: Echo $ variable name set (all variables: including custom variables and environment variables)
Cancel variable: unset variable name
Scope: Valid only in the current shell

Example:

2. Environment variables
Defining environment Variables: Method One export url2=test2.3glong.com
Method two export URL1 convert a custom variable to an environment variable
Reference environment variable: $ variable name or ${variable name}
View environment variables: ECHO $ variable name env For example Env |grep BACK_DIR2
Cancel environment variable: unset variable name
Variable scope: Valid at current shell and child shell
Example:

[[email protected] ~]# url1=test.3glong.com             #定义自定义变量 [[email protected] ~]# export url2=test2.3glong.com  # Defining environment Variables [[email protected] ~]# /bin/bash               #再开启一个bash进入子shell [[email protected] ~]# pstreeinit─┬─abrtd      ├─acpid     ├─anacron     ├─atd      ├─auditd───{auditd}     ├─console-kit-dae───63*[{ console-kit-da}]     ├─crond     ├─cupsd      ├─dbus-daemon     ├─dhclient      ├─hald─┬─hald-runner─┬─hald-addon-acpi     │      │              └─hald-addon-inpu     │      └─{ hald}     ├─login───bash     ├─master─┬─pickup      │        └─qmgr     ├─5*[ mingetty]     ├─rpc.statd     ├─rpcbind      ├─rsyslogd───3*[{rsyslogd}]     ├─sshd───sshd───bash───bash───pstree     #确认处于子shell      ├─udevd───udevd      └─xinetd[[email protected] ~]# echo  $url 1             #输出自定义变量                                                                     #发现什么都没有 [[email protected] ~]# echo  $url 2test2.3glong.com                                    #环境变量输出了 [[email protected] ~]# exit                           # Exits the current sub-shellexit[[email protected] ~]# echo  $url 1test.3glong.com                                        #自定义变量只能当前shell使用 [[ Email protected] ~]# echo  $url 2test2.3glong.com[[email protected] ~]# export url1                 #转换i自定义变量为环境变量 [[email protected]  ~]# /bin/bash[[email protected] ~]# echo  $url 1                 #进入子shelltest .3glong.com                                         # The child shell is normal.

3. Position variables
$ $4 $ $6 $7 $8 $9 ${10}

[[email protected] ~]# vim test.sh echo "2nd position parameter is $" echo "1th positional parameter is" echo "the 4th position parameter is $4" echo "All parameters are: $*" echo " All parameters are: [email protected] The number of "echo" parameters is: $# The PID of the current process is: $$ "Echo ' $1= ' $1echo ' $2= ' $2echo ' $3= ' $3echo ' $*= ' $*echo ' [emai L protected]= ' [email protected]echo ' $#= ' $ #echo ' $$= ' $$

4. Pre-defined variables
The name of the foot
$* all the parameters
[Email protected] all the parameters
Number of $# parameters
$$ PID of the current process
$! PID of the previous background process
$? The return value of the previous command 0 indicates success
Considerations when defining or referencing variables:
"" Weak reference
' Strong references '

[Email protected] ~]# School=emlog[[email protected] ~]# echo "$school blog" emlog blog[[email protected] ~]# echo ' $schoo L Blog ' $school blog

This article is from the "Dragon Blog" blog, please be sure to keep this source http://3glong.blog.51cto.com/11271871/1782248

SHELL variables for the collation of notes