Linux Shell control statements

Control statement

The following three methods are commonly used. A lower-case string represents a control command, while a lower-case string represents other meanings.
1. If test then commands Elif test then commands fi
2. for VAR in list do commands done
3. While test do commands done

Commands refers to the command set; var refers to the variable; test is a test statement, there are several forms:

View code

 1   [  -  A file] If the file exists, it is true.
  2   [  -  B file] If the file exists and is a special block file, it is true.
  3   [ -  C file] If the file exists and is a special word file, it is true.
  4   [  -  D file] If the file exists and is a directory, it is true.
  5   [  -  E file] If the file exists, it is true.
  6   [  -  F file] If the file exists and is a normal file, it is true.
  7  [  -  G file] If the file exists and the sgid has been set, it is true.
  8   [  -  H file] If the file exists and is a symbolic connection, it is true.
  9   [  -  K file] It is true if the file exists and the stick bits have been set.
  10   [  -  P file] If the file exists and is a name pipe (f If O), it is true.
 11   [  -  R file] true if the file exists and is readable.
  12   [  -  S file] If the file exists and the size is not 0, it is true.
  13   [  -  T FD] It is true if the file descriptor FD is open and points to a terminal.
  14   [  - U file] If the file exists and SUID (  Set  User ID) is true.
  15   [  -  W file] If the file exists and is writable, it is true.
  16   [  -  X file] true if the file exists and is executable.
  17   [  -  O file] If the file exists and is a valid user ID, it is true.
 18   [  -  G file] If the file exists and belongs to a valid user group, it is true.
  19   [  -  L file] If the file exists and is a symbolic connection, it is true.
  20   [  -  N file] If file exists and has been mod, it is true if IED since it was last read.
  21   [  - S file] If the file exists and is a socket, it is true.
  22   [File1  -  NT file2] If file1 has been changed more recently than file2, or if file1file2 does not is true. Exists and
  23   [File1  -  Ot file2] True if file1 is older than file2, or file2 exists and file1 does not exist.
  24   [File1  -  EF file2] True if file1 and file2 point to the same device and node number.
 25   [  -  O optionname] true if the shell option "optionname" is enabled.
  26   [  -  Z string] If the length of string is zero, it is true.
  27   [  -  The length of N string] or [String] string is non-zero.  -  Zero is true.
  28  [String1  =  String2] If the two strings are the same.  =  May be used instead  =     For  Strict POSIX compliance is true.
  29   [String1  ! =  String2] true if the strings are not equal.
  30   [String1  < String2] If string1 sorts before string2 lexicographically  In  The current locale is true.
  31   [String1  >  String2] If string1 sorts after string2 lexicographically  In  The current locale is true.
  32   [Arg1 op arg2] OP  Is  One  - EQ,  -  Ne,  -  LT,  -  Le,  -  GT or  -  Ge.
  33   
  34   [  !  Expr] True if expr is false.
 35   [(Expr)] returns the value of expr. This can be used to ignore normal operator priorities.
  36   [Expr1  -  A expr2] True if expr1 and expr2 are all true.
  37   [Expr1  -  O expr2] True if expr1 or expr2 is true.
  38   
  39   The test command can also be returned. For example
  40    If     "  Hello World  "     |  Grep hello  >     /  Dev  /  Null  ; Then ECHO  "  Hello is here "  ; FI 

4. Example of if:

 
If [$ count-lt $ times] Then count = 'expr $ count + 1' echo $ count fi
 

5. For example:

 
For Loop in 'LS' do echo $ loop done
 

6. Example of while:

 
While [$ count-lt $ times] do count = 'expr $ count + 1' echo $ count done
 

Notes
1. Be careful with some spaces. For example, in if ["$ Str" = "ABC"], spaces before and after [and] cannot be removed.
2. Expression of the variable. By default, shell is executed by string. Therefore:
A = 1
A = $ A + 1
Then a becomes the string "1 + 1", not 2. If he runs in mathematics, use the expr command.
A = 1
A = 'expr $ A + 1'
Note that the leading and trailing spaces cannot be removed here. Otherwise, "$ A +" will be seen as a whole, and all parsing errors will occur.