03. Regular Files

Topics:
  • Files
  • Commands
    • cat
    • more and less
    • ls
    • touch
    • cp
    • mv
    • rm
  • Wild Cards or File name Expansion

Files
  • Data on computers are stored in units called files
  • Files are identified by their name and their location in the system
  • Filename requirements for Linux:
    • can be any combination of text characters: letters, numbers, and some punctuation marks
    • case sensitive: uppercase and lowercase letters are different
    • cannot contain the following characters: &  ;  |  *  +  ?  ~  !  $  ^  #  /  \  ‘  “  `  [  ]  (  )  {  }  <  >  and space
  • Filename recommendations:
    • make filename descriptive, use mostly letters and numbers
    • can be multiple words, separate words by  _  or  –
    • don’t need extensions.  Use filename extension if it helps make the filename descriptive
    • most filenames should not start with ". "  A period at the front of the filename indicates that it’s a hidden file, which means the file will not show up when you do a default listing of files

cat
  • cat: (for concatenate) displays on screen the content of a file
  • Common format:   cat  filename
  • cat can also accept a file list made of multiple filenames, separated by space. If given a file list, cat will list the content of one file after another, in the order given on the command line 
  • cat can also accept no argument. If there is no argument, cat will echo what you type on screen and stay in display mode. To get back to the shell prompt, use control-d

more and less
  • more: displays to screen the content of a file, page by page
  • less: displays to screen the content of a file, page by page
    • less is newer than more. It is more efficient to use less when you have a large file to look at
  • Common format:     more  filename    or     less  filename
  • more / less will display the first page of the file
    • To display the next page:     space
    • To display the previous page:  control-b or b
    • To quit:     q
  • more / less can also accept a file list.  The files in the list will be displayed one by one, in the order on the command line

ls
  • ls: (for list) will list the filename of all files in the directory
  • Common format:    ls  
    • All non-hidden filenames in the current directory are displayed in alphabetical order
  • Common format:    ls   filename
    • If the file exists, the filename is echoed back on screen.  If the file doesn’t exist, an error message is displayed. This is a quick way to check whether a file exists or not
    • ls will also accept a file list. Each filename in the list that exists will be listed, or an error message will appear for each filename that doesn’t exist
  • Common format:    ls  –a
    • (option a is for all) Shows all filenames in the directory, including hidden filenames
  • Common format:   ls  -l
    • (l for long) Long listing of filenames. Each file and its attributes are listed on a separate line, in column format
    • The order of the attributes are:
      • Mode: shows file type and access rights
      • Number of hard links
      • Owner ID
      • Group ID of owner
      • Size: in bytes
      • Last access time / date
      • Filename
  • The arguments and options can be combined, such as - ls  -l  fileA    or     ls -al

touch
  • Common format:    touch  filename
    • If filename doesn’t exist, then a new, empty file with the given name is created
    • If filename exists, the access time of the file will be updated to the current time
  • touch can also accept a file list.  Each filename in the list will either be created or have its access time updated
  • touch is useful to quickly create new files

cp
  • cp: (for copy) will copy the content of a source file to the destination file. The source_file still exists after copying.
  • Common format:   cp   source_file    destination_file
    • The 2 arguments are required, in the order shown
    • If the source_file doesn’t exist, cp will send to screen an error message
    • If the destination_file doesn’t exist, cp will create a new destination file which is a copy of the source_file
    • If the destination_file exists, its content will be overwritten by the source_file
  • To have the system ask for confirmation before overwriting an existing file:    cp  -i  source_file    destination_file  
    • the option i is for interactive
    • you answer y for overwriting, n for not overwriting. When not overwriting, there is no copying done

mv
  • mv: (for move) will move the content of a source file to the destination file, and then delete the source_file. This effectively renames the source file to the destination filename
  • Common format:   mv   source_file    destination_file
    • The 2 arguments are required, in the order shown
    • If the source_file doesn’t exist, mv will send to screen an error message
    • If the destination_file doesn’t exist, mv will create a new  destination file
    • If the destination_file exists, its content will be overwritten by the source_file
  • To have the system ask for confirmation before overwriting an existing file:    mv  -i  source_file    destination_file  
  • the option i is for interactive
  • you answer y for overwriting, n for not overwriting the existing file

rm
  • rm: (for remove) will delete a file
  • Common format:    rm  filename       
    • where filename is required
    • rm will also accept a file list, where each of the file in the list will be deleted
  • To have the system ask for confirmation before deleting the file:     rm  –i  filename 
    • The i option is for interactive
    • You answer y for delete, n for not delete   

Wildcards or Filename Expansion
  • Some of the commands working with files will accept a file list. One way to enter a file list is by typing each filename individually, separated by space. An easier way is by using wildcards.
  • Wildcard characters are used in a filename, in place of or in addition to the characters in the filename. 
  • Each wildcard in a filename is interpreted by the shell to match a set of characters, rather than one single character. Thus the shell expands the wildcard character in a given filename to match any number of existing filenames. 
  • Wildcard characters:
    • ?    matches any one character
    • *     matches 0 or more of any character
    • [ character set ]    matches any one character in the character set
  • Examples of using ?
    • The name lab? will match any filename that has lab and 1 more character. For example, it can match the actual filenames lab1, lab2, labA, or labs, but will not match lab12 (2 characters after lab), lab (no character after lab), or laboratory (too many characters after lab)
    • The name my???file will match any filename that starts with my, followed by any 3 characters, and ending with file
    • The ? can be used one after another to specify a certain number of characters in the filename
  • Examples of using *
    • The name lab* will match any filename that has lab and 0 or any number of characters after. It can match lab, lab2, lab12345, lab_report, labs, laboratory
    • The name my*file will match any filename that starts with my and ends with file, regardless of any number of characters in between
    • The * is not used one after another since one * is expanded to as many characters as needed already
  • Examples of using [character set]
    • The characters in between the [ ] means that there is a match if the filename contains one of the characters in the set 
    • The wildcard lab[123] will match the filename that has lab followed by the number 1 or 2 or 3. It can match the actual filenames lab1, lab2, or lab3 but will not match lab123 or lab12
    • The characters in the set can be described with a short hand notation, if they follow one after another in order: [a-z] matches 1 lowercase letter of any kind, [2-8] matches 1 digit that can be 2,3,4,5,6,7, or 8
    • The name my[23][xy]file will match the filenames my2xfile, my2yfile, my3xfile, or my3yfile only
    • The [character set] can be used one after another to specify different groups of allowable characters
    • Each of the 3 wildcards can be used separately or together in the filename. They can also be used by themselves or with specific text characters in the filename

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