Shell learning notes and shell scripts

Shell learning notes and shell scripts
1. determine the format

1) test parameter file

Example: test-e/root/install. log

2) [parameter file] -- recommended

Example: [-e/root/install. log]

Note: There must be spaces at the end of the brackets and at the front.

2. Determine file type parameters

1)-d file: determines whether the file exists and is a directory file

2)-e file: determines whether the file exists

3)-f file: determines whether the file exists and is a common file

4)-s file: determines whether the file exists and is not empty

5) other file types:

-Block B device files;-c character device files;-L symbol link files;-p pipe files;-S socket files

Example:

[root@localhost ~]# [ -d /root ] && echo yes || echo noyes[root@localhost ~]# [ -e /root/install.log ] && echo yes || echo noyes[root@localhost ~]# [ -f /root/install.log ] && echo yes || echo noyes[root@localhost ~]# [ -s /root/install.log ] && echo yes || echo noyes

 

3. Determine File Permission Parameters

1)-r file: determines whether the file exists and has read permission

2)-w file: determines whether the file exists and whether it has the write permission

3)-x file: determines whether the file exists and has the execution permission.

4) Other File Permission judgment:

-U SUID permission;-g SGID permission;-k SBit permission

Example:

[root@localhost ~]# [ -r /root/install.log ] && echo yes || echo noyes[root@localhost ~]# [ -w /root/install.log ] && echo yes || echo noyes[root@localhost ~]# [ -x /root/install.log ] && echo yes || echo nono

 

4. Comparison of Two files

1) file 1-nt file 2: Determine whether the modification time of file 1 is newer than that of file 2

2) file 1-ot file 2: Determine whether the modification of file 1 is earlier than that of file 2.

3) file 1-ef file 2: Determine whether file 1 is consistent with the lnode number of file 2. It can be understood that two files are the same file. This judgment is a good method for judging hard links.

Example:

[root@localhost ~]# [ /root/install.log -nt /root/install.log.syslog ] && echo yes || echo noyes[root@localhost ~]# [ /root/install.log -ot /root/install.log.syslog ] && echo yes || echo nono[root@localhost ~]# [ /root/install.log -ef /root/install.log.syslog ] && echo yes || echo nono

 

5. Comparison of two integers

1) integer 1-eq integer 2: judge whether it is equal

2) integer 1-ne integer 2: determines if it is not equal

3) integer 1-gt integer 2: determines whether the value is greater

4) integer 1-lt integer 2: judge whether it is smaller

5) integer 1-ge integer 2: determines whether the value is greater than or equal

6) integer 1-le integer 2: judge whether it is less than or equal

Note: In shell, all variables are of the character type, but when an integer comparison parameter is added, the variables on both sides are considered to be of the integer type.

Example:

[root@localhost ~]# [ 1 -eq 1 ] && echo yes || echo noyes[root@localhost ~]# [ 1 -ne 1 ] && echo yes || echo nono[root@localhost ~]# [ 2 -gt 1 ] && echo yes || echo noyes[root@localhost ~]# [ 2 -lt 1 ] && echo yes || echo nono[root@localhost ~]# [ 1 -ge 1 ] && echo yes || echo noyes[root@localhost ~]# [ 1 -le 1 ] && echo yes || echo noyes

 

6. String judgment

1)-z string: determines whether the string is null

2)-n string: determines whether the string is not empty

3) string 1 = string 2: judge whether two strings are equal

4) string 1! = String 2: judge whether two strings are not equal

Example:

[root@localhost ~]# str="abc"[root@localhost ~]# [ -z $str ] && echo yes || echo nono[root@localhost ~]# [ -n $str ] && echo yes || echo noyes[root@localhost ~]# str2="efg"[root@localhost ~]# [ "$str" == "$str2" ] && echo yes || echo nono[root@localhost ~]# [ "$str" != "$str2" ] && echo yes || echo noyes

 

7. Multi-condition judgment

1) judgment 1-a judgment 2: represents the logic and, equivalent to and

2) 1-o judgment 2: logical or, equivalent to or

3 )! Judgment: indicates that the logic is not, which is equivalent to not

Example:

[root@localhost ~]# [ 1 -eq 1 -a 1 -eq 2  ] && echo yes || echo nono[root@localhost ~]# [ 1 -eq 1 -o 1 -eq 2  ] && echo yes || echo noyes[root@localhost ~]# [ ! 1 -eq 2  ] && echo yes || echo noyes