File Handling in Unix

1
Chapter 3

• File Handling in Unix

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What is a file

• A file is a stream of bytes.
• A file is a stream of ASCII bytes.
• A directory is a file
• Input devices read streams of ASCII bytes
• Output devices write streams of ASCII btyes

3
pwd

• Print Working Directory shows you where you are
  in the file system
• Useful when you are trying to do something you
  may not be allowed to doso you can see why you
  dont have permission.

4
mkdir

• Used to create directories
• You can only create directories in file systems
  you have permission to write to
• A nice set of directories makes it easy to
  organize your files

5
cd

• Change directory
• Allows you to change your current working
  directory to any other you have permission to
  enter
• cd by itself sends you to your home dir
• the address can be relative or absolute

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relative addressing

• The address must not start with a /
• the current address is applied to the left of the
  address specified
• is often a shorter way to specify an address
• two useful relative addresses
• dot (.) identifies the current working directory
• dot dot (..) identifies the parent directory of
  the current working directory

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absolute addressing

• The address must start with a /
• the address must be the complete path to the file
• some absolute addresses are shorter than using
  relative addressing, especially when changing
  file systems

8
touch

• touch filename will create the filename if it
  does not exist
• touch filename will change the date last accessed
  if the file exists
• you can specify a date that touch is to use with
  the -t option

9
ls

• Used to list one or more file names in the
  current directory
• can show many different data elements about a
  file
• Understanding the long file listing is very
  important

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ls -l listing

• Leftmost byte is file type
• d is a directory file
• - is a regular file
• l is a link

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permissions mask

• First 3 bytes - User
• The user owns the file
• r (read)
• w (write)
• x (execute)
• - permission denied
• 2nd 3 bytes - Group
• 3rd 3 bytes - Other

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ls -l listing

• Link Count - number of links to the files inode
• User Name - the login ID of the user
• Users Group - the group from /etc/passwd
• File Size - Either in bytes or blocks
• Creation/Modified Date - can be altered by touch
• File Name - the actual link name

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file names

• There is no required formatting for Unix file
  names.
• Most systems allow at least 256 byte file names
• While you can use special characters in file name
  it is a Bad Thing to do so.
• If you must use special characters you must
  protect them from interpretation by the shell

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dots and file names

• To the shell, the dot is just another character
  in a file name unless it is the first character.
• A file name may have many dots
• a.file.name.with.lots.of.d.o.t.s. is just fine
• The only usual tool that requires a dot in the
  file name is the C compiler which wants prog.c as
  input.

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dotted files

• A filename that starts with a dot is called a
  dotted file, a hidden file or a protected file.
• dotted files are exempt from file matching
  metacharacter expansion.
• To see dotted files, use the -a (all) option on
  the ls command

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cp

• Two forms
• cp sourcefile targetfile
• copies (renames) a single file
• cp sourcefiles targetdirectory
• copies a set of files to a directory
• second argument must be a directory
• The -i (inquire/interactive) option can prevent
  you from overwriting existing files.

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cpsome useful options

• -i (inquire, keep from overwriting existing
  files.)
• -r (recursive copy the entire directory structure
  from here on down)
• -v (verbose show what is happening)
• The one who makes the copy owns the file.

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mv

• Also called rename, changes the path and/or name
  of a file.
• Does not change the ownership or permissions.
• Can move whole directories
• the -i option is very useful!

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cat

• Concatenation (of two or more files or a single
  file and standard out.)
• Often used to view short files
• Can be used to combine files with redirection of
  output
• Can also be used to create short files

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more

• Designed to display a file one page at a time
• Some commands
• ltspacegt display next full page of text
• ltreturngt display next line of text
• q quit more
• / ltpatterngt search for pattern in file
• . (dot) repeat previous command

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less

• Created to give more functionality to more
• Allows the use of arrow keys to move
• more commands also work
• Remember, less is more! ?

22
rm

• Removes a link to an inode
• when the inode count drops to zero, the file is
  deleted
• Couple of important options
• -i prompts before each removal
• -r sets up a recursive remove, removes everything
  from the current directory on down this can be a
  Bad Thing!

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rmdir

• Designed to remove empty directories
• wont remove directory if it contains files
• can use rm -r to remove a directory and the files
  it contains
• Once a file or directory has been deleted by the
  file system, you cant get it back.

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The Structure of /etc/passwd

• You should memorize the 7 fields in /etc/passwd
• This file is critical for many Unix tasks
• It is the file consulted for login, to apply file
  permissions, etc.