Shulou(Shulou.com)06/03 Report--

View system variables:

1.env command

[root@localhost ~]# env

2. Set command

[root@localhost ~]# set

*set can display user-defined variables

Custom variables:

1. Definition of variables:

[root@localhost ~]# a=test[root@localhost ~]# echo $atest

2. Variable naming rules: can contain upper and lower case letters, numbers, underscores (cannot start with numbers)

[root@localhost ~]# a=1[root@localhost ~]# echo $a1[root@localhost ~]# a_1=2[root@localhost ~]# echo $a_12[root@localhost ~]# a1=3[root @ localhost ~]# echo $a13[root@localhost ~]# 1a=4-bash: 1a=4: Command not found

3. Single quotes are required when variable values contain special characters ($ / \ #spaces, etc.):

[root@localhost ~]# a=abc[root@localhost ~]# echo $aabc[root@localhost ~]# a='a b c'[root@localhost ~]# echo $aa b c[root@localhost ~]# a=a b c-bash: b: no command found

4. Accumulation of variables: when the variable value contains the variable name, double quotes are required to read the variable value, and the variable name is recognized as a string with single quotes.

[root@localhost ~]# a=1[root@localhost ~]# b=2[root@localhost ~]# echo $a$b12[root@localhost ~]# c='$a$b'[root@localhost ~]# echo $c$a$b[root@localhost ~]# c="$a$b"[root@localhost ~]# echo $c12

5. Global variables: variables defined in the current shell (terminal) only take effect in the current shell (terminal), using global variables can make variables also take effect in the child shell of the current shell, but global variables defined in the child shell will not take effect in the parent shell (global variables can only take effect in the current shell and the child shell of the current shell)

Define global variables Command: export

[root@localhost ~]# a=test[root@localhost ~]# echo $atest[root@localhost ~]# bash[root@localhost ~]# echo $a[root@localhost ~]# exitexit[root@localhost ~]# export a=test[root@localhost ~]# bash[root@localhost ~]# echo $atest

6. Delete variable: unset variable name

[root@localhost ~]# a=1[root@localhost ~]# echo $a1[root@localhost ~]# unset a

Environmental variables:

1. Variable configuration file:

a. System level: /etc/profile,/etc/bashrc

b. User level: ~/.bash_profile,\~/.bashrc,\~/.bash_history,\~/.bash_logout

* System-level profiles are usually loaded at login time, user-level profiles only work for a single user

PS1 variable: indicates the content at the beginning of each line ([root@localhost ~]#)

[root@localhost ~]# echo $PS1[\u@\h \W]\$

*u for user, h for hostname, W for current directory

Change upper case W to lower case W to display absolute path:

[root@localhost ~]#cd /etc/sysconfig/[root@localhost sysconfig]#PS1='[\u@\h \w]\$'[root@localhost /etc/sysconfig]#

PS2 variable: (used in another mode, such as after logging into mysql)

[root@localhost ~]#echo $PS2>

cut command:

-d parameter: Specify division symbol, -f parameter: Specify number of segments, -c parameter: Specify number of characters

[root@localhost ~]#cat 1.txtroot:x:0:0:root:/root:/bin/bash[root@localhost ~]#cat 1.txt |cut -d ":" -f 1root[root@localhost ~]#cat 1.txt |cut -d ":" -f 1,2root:x[root@localhost ~]#cat 1.txt |cut -d ":" -f 1-5root:x:0:0:root[root@localhost ~]#cat 1.txt |cut -c 4t

sort command: sort the contents of each line from ASCII to large

[root@localhost ~]#[root@localhost ~]#cat 1.txt abcaa_1aabbop#test

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