Shell learning notes-conditional attention

Shell learning notes-conditional attention
1. 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. file type parameter 1)-d file: determines whether the file exists and is a directory file 2)-e file: determines whether the file exists 3)-f file: determine whether a file exists and whether it is a common file 4)-s file: Determine whether the file exists and whether it is not empty. 5) other file types:-B device files; -c character device file;-L symbol link file;-p pipeline file;-S socket file 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. file Permission parameter 1)-r file: determines whether the file exists and has read permission 2)-w file: determines whether the file exists and whether the file has write permission 3) -x file: determines whether the file exists and whether the file has the execution permission. 4) other file permissions:-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. compare two files 1) file 1-nt file 2: Determine whether the modification time of file 1 is later than that of file 2 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. two integers: 1) integer 1-eq integer 2: judge whether it is equal 2) integer 1-ne integer 2: judge whether it is not equal 3) integer 1-gt integer 2: judge whether it is greater than 4) integer 1-lt integer 2: judge whether it is less than 5) integer 1-ge integer 2: judge whether it is greater than or equal to 6) integer 1-le integer 2: judge whether it is less than or equal. Note: In shell, all variables are character types, but when an integer comparison parameter is added, the variables on both sides are considered as integer types. 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: judge whether the string is null 2)-n string: Determine whether the string is not null 3) string 1 = string 2: Determine whether two strings are equal 4) string 1! = String 2: Example of determining whether two strings are not equal: [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 logic and, equivalent to and 2) judgment 1-o judgment 2: represents logic or, equivalent to or 3 )! Judgment: indicates the logic is not, which is equivalent to the 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