Manipulating files in UNIX

1
Manipulating files in UNIX
2
Common operations of files

• Common operations
• We will learn to do these operations and
  more....

• Create a file
• Print a file
• Delete a file
• Rename a file
• Move a file from one directory into another
  directory

3
Identifying a file

• We need to identify a file before we can perform
  an operation (like delete) on the file.
• A file can be identified using

• An absolute (file) path, or       
• An relative (file) path

4
Absolute file path

• Absolute file path

• An absolute file path tells the computer how to
  find a file starting from the root directory
• An absolute file path of a particular file x
  consists of a list of directory names starting
  from the root directory "/" to the directory
  containing the file x and then followed by the
  name of the file (x)
• The list of names is separated by the "/" symbol
  

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Absolute file path (cont.)
The path for the indicated file (myFile2) is

• /home/cheung/cs170/myFile2

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Relative file path

• Relative file path

• An relative file path tells the computer how to
  find a file starting from the current directory
• An relative file path of a particular file x
  consists of a list of directory names starting
  from the current directory to the directory
  containing the file x and then followed by the
  name of the file (x)
• The list of names is separated by the "/" symbol

7
Relative file path (cont.)

• Example 1
• If the current directory is /home/cheung, then
  the path for the indicated file (myFile2) is

• cs170/myFile2

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Relative file path (cont.)
If the current directory is /home/cheung/cs170,
then the path for the indicated file (myFile2)
is

• myFile2

9
Relative file path (cont.)

• Advice

• When working on files, always change the working
  directory to the one that contains the files.
• It will save you a lot of key strokes (typing)

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Common operations on files

• Common operations on files

• Create a file
• Print a file to the terminal
• Print a file to the printer
• Delete a file
• Rename a file
• Move a file from one directory (folder) to
  another directory (folder)

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Create a file

• An electronic file is created using a computer
  application (program) called an editor
• (An editor in computer lingo is a program !!!)
• Some commonly used editors that you have used on
  a PC
• We will learn to use gedit (GNU editor) in
  another webnote.

• Microsoft Word
• Notepad

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Print the content of a file out to the terminal

• The command ( application) that is used to print
  the content of a file is
• (cat is an abbreviation of the word catenate)

cat FILE-PATH
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Print the content of a file out to the terminal
(cont.)

• Example

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Catenating multiple files

• The cat command can catenate an arbitrary number
  of files to the terminal
• Example catenate myFile1 and myFile2 to the
  terminal

15
Redirecting the input and output to a file

• In UNIX, the output that an application prints to
  the terminal, can be stored to a file
• Also, the input that an application reads from
  the keyboard, can be read from a file. This
  feature is called

• Input/Output redirection (or IO redirection for
  short)

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Redirecting the input and output to a file (cont.)

• Redirecting the output to a file

• The output of any UNIX command can be sent to a
  file
• by adding " gt FileName" to the command
• In other words

• any UNIX command gt FileName

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Redirecting the input and output to a file (cont.)

• Example redirecting the output of "ls" into a
  file

18
Redirecting the input and output to a file (cont.)

• Example redirecting the output of cat myFile1
  myFile2 to a file (named myFile3)

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Redirecting the input and output to a file (cont.)

• You can see that myFile3 contains the catenation
  of the files myFile1 and myFile2
• So the cat command can be used to catenate
  multiple files together !!!
• (Hence the name cat)
• We will learn about input re-direction at a later
  lecture

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Print a file to the printer

• Use this command to print a file to the default
  printer
• When you print a file from a computer in the
  MathCS lab, the default printer is the printer
  located inside the area where the Lab
  assistant(s) sits

lpr File-Path
21
Print a file to the printer (cont.)

• Example
• will print the file named myFile1 in the current
  directory to the printer

lpr myFile1
22
Delete a file

• The command (application) used to delete a file
  is
• The word rm is an acronym for remove
• All files with names matching the File-Path will
  be removed

rm File-Path
23
Delete a file (cont.)

• Example

rm myFile1 (will delete the file named "myFile1"
in the current directory) rm
/home/cheung/cs170/myFile1 (will delete the file
"myFile1" in the directory/home/cheung/cs17
0)
24
Delete a file (cont.)

• Important note

• Make sure that your current directory is the
  correct one when you use a relative path with all
  UNIX commands !!!

25
Recovering deleted files in UNIX

• Very important

• When you delete a file (with rm, the file is
  really deleted in UNIX
• What I mean is the file is not moved into a
  trash directory
• (That's what happens in Microsoft Windows)
• In other words your file is gone forever

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Recovering deleted files in UNIX

• Restoring a file with a backup version

• Every night, all files in a UNIX system is saved
  (backup)
• If you deleted a file, you can recover an older
  version of the file as of yesterday
• In other words any work you do after the backup
  was made, will be lost....
• Send an email to help_at_mathcs.emory.edu if you
  need to recover a file using a backup copy.

27
Rename a file

• The command (application) used to rename a file
  with name old-File-Path to the name new-File-Path
  is

mv old-File-Path new-File-Path
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Rename a file (cont.)

• Important

• The new-File-Path file must not exist otherwise,
  2 things can happen
• If the new-File-Path exists and it is the name of
  a file, then the mv will report an error (and
  will not rename the file)
• If the new-File-Path exists and it is the name of
  a directory, then the mv will move the file
  old-File-Path into the directory new-File-Path

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Rename a file (cont.)

• Example
• will rename the file named myFile2 to the new
  name decl-of-indep

mv myFile2 decl-of-indep
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Rename a file (cont.)

• Illustrated
• Notice that after the file is renamed, the
  content of the file remains unchanged !

31
Move a file from one directory (folder) to
another directory (folder)

• The command (application) used to mv a file with
  name file-Path into the directory dir-Path is

mv file-Path dir-Path
32
Move a file from one directory (folder) to
another directory (folder) (cont.)

• Important

• This is in fact the same command for renaming a
  file
• The difference is dir-Path must be the path of
  an existing directory.
• (If dir-Path, the command will perform a rename
  operation !!!)

33
Move a file from one directory (folder) to
another directory (folder) (cont.)

• Illustrated

mv command with non-existing directory
mv command with existing directory