Linux-bash variable, bash script

Bash Variable type:
Environment variables
Local variables (local variables)
Positional variables
Special variables

Local variables:
Set Varname=value: scope for the entire bash process;

Local variables:
Local Varname=value: Scope is the current code snippet;

Environment variable: The scope is the current shell process and its child processes, (the action variable itself uses VARNAME, the value of the action variable is only $varname)
Export Varname=value
Varname=value
Export VARNAME
Export

[Email protected] ~]# export name
[Email protected] ~]# bash
[Email protected] ~]# echo $name
Cxiong
[Email protected] ~]# bash
[Email protected] ~]# echo $name
Cxiong

Reference variable: ${varname}, parentheses can sometimes be omitted

[Email protected] ~]# Name=cxiong
[Email protected] ~]# echo $name
Cxiong
[Email protected] ~]#

[Email protected] ~]# echo "My name is $name"
My name is Cxiong
[Email protected] ~]#

Positional variables:
$, $, ...

Special variables:
$?: The execution status return value of the previous command;

[[email protected] ~]# echo $?
0
[Email protected] ~]# Ll/varr
Ls:cannot access/varr:no such file or directory
[[email protected] ~]# echo $?
2
[Email protected] ~]#

program execution, there may be two types of return values:
Program execution Results
Program Status return codes (0-255)
0: Correct execution
1-255: Error execution, 1,2,127 system reservation;

Output redirection:
>
>>
2>
2>>
&>

Undo Variable:
Unset VARNAME

[Email protected] ~]# unset name
[Email protected] ~]# echo $name

[Email protected] ~]#

To view variables in the shell:
Set

[[Email protected] ~]# set
Bash=/bin/bash
Bashopts=checkwinsize:cmdhist:expand_aliases:extquote:force_fignore:hostcomplete:interactive_comments:login_ Shell:progcomp:promptvars:sourcepath
Bash_aliases= ()
Bash_argc= ()
bash_argv= ()
Bash_cmds= ()

To view environment variables in the current shell:
Printenv
Env
Export

Script: Command stack, according to the actual needs, combined with the command flow control mechanism to implement the source program

Shebang: Magic number
#!/bin/bash
# comment lines, do not execute

The kernel can only perform elf types

[Email protected] ~]# Nano first.sh
[email protected] ~]# ll first.sh

[Email protected] ~]# chmod +x first.sh
[Email protected] ~]# pwd

[Email protected] ~]# first.sh

#
#/etc/fstab
# Created by Anaconda on Sun June 21 02:15:00 2015
#
# Accessible filesystems, by reference, is maintained under '/dev/disk '
# See mans Pages Fstab (5), Findfs (8), mount (8) and/or Blkid (8) for more info
#

/dev/null: Software device, bit bucket, data black hole

$?

[[email protected] ~]# ID student &>/dev/null
[[email protected] ~]# echo $?
1
[[email protected] ~]# ID rhel &>/dev/null
[[email protected] ~]# echo $?
0
[Email protected] ~]#

The script starts a child shell process when it executes;
Scripts that are started on the command line inherit the current shell environment variable;
Scripts that are automatically executed by the system (non-command-line startup) require a self-defined environment variable;

Linux-bash variable, bash script