1. Wildcard

1.1 effect on documents

• *: matches any 0 or more characters or strings, including empty strings

[root@VM-0-17-centos ~]# rm -f *.sh

• ?: Match any 1 character

[root@VM-0-17-centos ~]# ls ?.*
a.out
[root@VM-0-17-centos ~]# ls ????.*  # Can be used multiple times
info.txt  test.sh

1.2 action on file / character

• [abcd]: match any one character in brackets

[root@VM-0-17-centos ~]# ls [abcd].*
a.out

• [a-z]: match any one character in a-z in brackets

[root@VM-0-17-centos ~]# ls [a-c].*
a.out

• [! b-z]|[\^b-z]: does not match any one character in b-z in brackets

[root@VM-0-17-centos ~]# ls [!b-d].*
a.out
[root@VM-0-17-centos ~]# ls [^b-d].*
a.out

tips:

• If [a-z] is used, it must be ensured that it is continuous. For example, [c-a] will report an error

2. Special characters

2.1 path and location related

• ~: user home directory

[root@VM-0-17-centos ~]# cd ~
[root@VM-0-17-centos ~]# pwd
/root

• -: last user's path

[root@VM-0-17-centos etc]# cd -
/root
[root@VM-0-17-centos ~]#   # After displaying, it will jump to the path of the last user

• .: current directory
• ..: upper level directory

tips:

• echo $OLDPWD can return the last user's path, so cd - is actually cd $OLDPWD

2.2 quotation marks*

• Single quotation mark '': strong reference. When outputting, the contents in quotation marks will be output as they are, and the commands in them will not be executed

[root@VM-0-17-centos ~]# echo '`date`'
`date`

• Double quotation mark "": weak reference. If the contents of quotation marks contain commands (backquotation marks are required), variables, escape characters, etc., these contents will be parsed before output

[root@VM-0-17-centos ~]# echo "`date`"
Sat Feb 19 16:42:06 CST 2022

• Backquote ` `: refers to a command, which is equivalent to $()
• No quotation marks: when spaces are encountered, the whole will be segmented. It is best to use double quotation marks

[root@VM-0-17-centos ~]# a=psj ps2
-bash: ps2: command not found

2.3 other special characters

• ;: Indicates the end of a command and is also a separator between commands

[root@VM-0-17-centos ~]# echo a;cat test.sh 
a  # echo a
#!/bin/bash  # cat test.sh

• #: the comment content, which is also the command prompt of the root user, is used as a separator in vim:

:%s#i#j#g  # Replace all i's in the text with j's

• $: indicates the contents of string variables. It is also a command prompt for ordinary users
• \: escape character / escape character, which restores the character with special meaning and also acts as a newline character
• |: pipe
• {}: generate sequence, which can also be used as the separation between reference variables and ordinary characters

[root@VM-0-17-centos ~]# echo {1..5}  # Equivalent to seq 5
1 2 3 4 5
[root@VM-0-17-centos ~]# echo ${a}2
psj2
[root@VM-0-17-centos ~]# echo {1..a}  # There is no continuous sequence from 1 to a, so it is output as a string
{1..a}

tips:

• The data stream (data content) transmitted by pipeline is not a file (name)
• seq 1 1 5 indicates the start point and the end point of the step

2.4 special characters in Bash

• &&: when connecting two commands, the former command is correct and the latter command will be executed. If the former error, the latter command will not be executed

[root@VM-0-17-centos ~]# echo a && cat test.sh 
a
#!/bin/bash
[root@VM-0-17-centos ~]# cho a && cat test.sh 
-bash: cho: command not found

• ||: when connecting two commands, if the previous command is wrong, the latter command will be executed. If the previous command is correct, the latter command will not be executed

[root@VM-0-17-centos ~]# echo a  || cat test.sh 
a
[root@VM-0-17-centos ~]# cho a  || cat test.sh 
-bash: cho: command not found
#!/bin/bash

• !: Indicates negation in bash; Mandatory in vim; Using it directly on the command line means finding a command that starts with a string and running it

[root@VM-0-17-centos ~]# !c
cho a  || cat test.sh   # Command line starting with c
-bash: cho: command not found  # Execute the command
#!/bin/bash