Detailed explanation of shell condition judgment

test conditional

test  command 
1.  Judgment of document type and existence 
1.1 -e		 Determine whether a file or directory exists 
[[email protected] ~]# test -e bbb.txt && echo ' file / directory already exists ' || touch bbb.txt 
[[email protected] ~]# test -e bbb.txt && echo ' file / directory already exists ' || touch bbb.txt 
 file / directory already exists 
[[email protected] ~]# test -e bbb && echo ' file / directory already exists ' || mkdir bbb 
[[email protected] ~]# test -e bbb && echo ' file / directory already exists ' || mkdir bbb 
 file / directory already exists 

1.2 -f		 Determine if it is a file 
[[email protected] ~]# test -f bbb.txt && echo 'bbb.txt It's a document '
bbb.txt It's a document 
[[email protected] ~]# test -f bbb || echo 'bbb It's not a document '
bbb It's not a document 
1.3	-d		 Determine whether it is a directory 
1.4	-b		 Judge whether it is block device device 

1.5	-c		 Judge whether it is character device device 
1.6	-S		 Judge whether it is socket file 
1.7	-P 		 Judge whether it is FIFO(pipe) file 
1.8	-L		 Determine whether it is a connection 

2.  Judgment on file permissions 
2.1	-r		 Determine whether   Can be read 
2.2	-w		 Determine whether 	 Can write 
2.2	-x		 Determine whether 	 Executable 
2.3	-u		 Judge whether there is  SUID  attribute 
2.4	-g		 Judge whether there is  SGID  attribute 
2.5	-k		 Judge whether there is  sticky bit  attribute 
2.6	-s		 Judge whether it is   Blank file 

3.  Comparison between two files , Such as  test file1 -nt file2
3.1	-nt		file1 Is it better than file2  new （new than）
3.2	-ot		file1 Is it better than file2  used （old than）
3.3	-ef		file1 Is it better than file2  Whether it is the same file 
#  Can be used for  hard link  The judgement of , As long as the meaning lies in determining whether both files point to the same inode

4.  Two   Integers   The judgment of the 
4.1	-eq		 equal 
4.2	-ne		 It's not equal 
4.3	-gt		 Greater than 
4.4	-lt		 Less than 
4.5	-ge		 Greater than or equal to 
4.6	-le		 Less than or equal to 

5.  Judge string data 
5.1	test -z str		 Determine whether it is an empty string ,str Empty return true
5.2	test -n str		 Determine whether the string is not empty ,str Not empty return true（PS;-n  It can be omitted ）
5.3	test str1 = str2		 Judge whether it is equal 
5.4	test str1 != str2		 Judge whether it is not equal 

6.  Multiple condition judgment 
6.1	-a		 Both are established at the same time （and）
test -r file -a -x file		#  The file also has  r  Authority and  x jurisdiction 
6.2	-o		 Either of the two is true （or）
6.3	!		 Take the opposite 

Bracket conditional judgment

The functions and test identical
Most used
The front and back must be separated by a space ：[ -n $name ]

1.  String judgment 
[ -z "str" ] 	str The length of is zero , It is true  , That is to judge whether it is empty , Empty is true ;
[ -n "str" ] 	str The length of is non-zero , It is true  , That is, judge whether it is non empty , Not empty is true ;
[ "str1" = "str2" ] 		 The two strings are the same , It is true ;
[ "str1" != "str2" ] 		 The two strings are different , It is true ;
[ "str1" ]　 				 String is not empty , It is true , And -n similar .

2.  File to judge 
[ -e file ]			file  There is , It is true .
[ -d DIR ]			file  There is , And it's a directory , It is true .
[ -f file ]			file  There is , And it's an ordinary file , It is true .
[ -r file ]			file  Exist and readable , It is true .
[ -w file ]			file Exists and is writable , It is true .
[ -x file ]			file  Existing and executable , It is true .

[ -b file ]			file  There is , And is a block special file , It is true .
[ -c file ]			file  There is , And it's a word special file , It is true .

[ -g file ] 		file  Exists and has been set SGID, It is true .
[ -k file ] 		file  The adhesive bit exists and has been set , It is true .
[ -p file ] 		file  There is and is a name pipeline (F If O), It is true .

[ -s file ] 		file  Exist and not of size 0, It is true .
[ -t FD ] 			 File descriptor  FD  Open and point to a terminal , It is true .
[ -u file ] 		file  Exist and set SUID (set user ID), It is true .

[ -O file ] 		file  Exists and is a valid user ID, It is true .
[ -G file ] 		file  Exists and belongs to a valid user group , It is true .
[ -L file ] 		file  Exists and is a symbolic connection , It is true .
[ -N file ] 		file  There is  and has been mod, If ied since it was last read It is true .
[ -S file ] 		file  Exists and is a socket , It is true .
[ file1 -ot file2 ]		file1  Than  file2  To the old ,  perhaps  file2  Exist and  file1  Do not save , It is true in .
[ file1 -ef file2 ] 	file1  and  file2  Point to the same device and node number , It is true .
[ $a -gt $b -a $a -lt $c ]		-a		 And 
[ $a -gt $b -o $a -lt $c ]		-o		 or 
[ ! -z ${name} ] 				!		 Take the opposite 	

Double bracket conditional judgment

[[ ]] Built in shell One of the orders in , It's better than just said [ ] Much more powerful . Support pattern matching in strings .
[ ] Out of commission && and ||, Only use -a and -o

[[email protected] ~]# [ -r "aaa.txt" && -w "aaa.txt" ] && echo "aaa.txt  Have read and write permission "
-bash: [:  The lack of  `]'
[[email protected] ~]# [[ -r "aaa.txt" && -w "aaa.txt" ]] && echo "aaa.txt  Have read and write permission "
aaa.txt  Have read and write permission 
[[email protected] ~]# [ -r "aaa.txt" -a -w "aaa.txt" ] && echo "aaa.txt  Have read and write permission "
aaa.txt  Have read and write permission 
[[email protected] ~]# 

When comparing strings, you can use the one on the right as a pattern （ This is the string on the right without double quotation marks . If the string on the right is enclosed in double quotes , It is considered to be a text string .）, Not just a string , such as [[ hello == hell? ]], The result is true .

Double bracket conditional judgment

let conditional