'$ {B-2}' and '$ {B:-2}' in Shell }"

In the shell studyCode

 #  !  /  Bin  /  Bash
# User can keyin filename to touch  3     New  Files.
Path  =  /  USR  /  Local  / Sbin:  /  USR  /  Local  /  Bin:  /  USR  /  Sbin:  /  USR  /  Bin:  /  Sbin:  / Bin:  /  USR  /  Games
Export path

Echo  "  I will use 'touch' command to create 3 files.  "  
Read  -  P  "  Please input the filename what you want:  "  Fileuser

Filename  = $ {Fileuser:  +  "  Filename  "  }

Date1  =  'Date  --  Date  =  '  2 days ago  '    + %  Y  % M  %  D'
Date2  =  'Date  --  Date  =  '  1 days ago  '    + %  Y  %  M  %  D'
Date3 =  'Date  + %  Y  %  M  %  D'

File1  =  "  $ Filename  ""  $ Date1  "  
File2  =  "  $ Filename ""  $ Date2  "  
File3  =  "  $ Filename  ""  $ Date3  "  

Touch $ file1
Touch $ file2
Touch $ file3 

In
Filename=$ {Fileuser:+"Filename"} For: + is not very clear, and then check the usage.

Lx @ Ubuntu :~ /Bsh/scripts $ B = "" lx @ Ubuntu :~ /Bsh/scripts $ c = "Z" lx @ Ubuntu :~ /Bsh/scripts $ echo a =a {A-1}, B =$ {B-2}, C =$ {C-3} A = 1, B =, c = zlx @ Ubuntu :~ /Bsh/scripts $ echo a =$ {A:-1}, B =$ {B:-2}, C =$ {C:-3} A = 1, B = 2, c = zlx @ Ubuntu :~ /Bsh/scripts $
 

 
+ Variant + | form meaning + ------------------------------------------------------------------------ + | $ {Variable :? Word} complain if undefined or null | $ {Variable:-word} use new value if undefined or null | $ {Variable: + word} opposite of the above | $ {Variable: = word} use new value if undefined or null, and redefine. + ---------------------------------------------------------------------- +