Manipulating files in UNIX

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Manipulating files in UNIX
Common operations of files
• Common operations:
• Create a file
• Print a file
• Delete a file
• Rename a file
• Move a file from one directory into another
directory
We will learn to do these operations and more....
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
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
Absolute file path (cont.)
The path for the indicated file (myFile2) is:
• /home/cheung/cs170/myFile2
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
Relative file path (cont.)
• Example 1:
If the current directory is /home/cheung, then the path for
the indicated file (myFile2) is:
• cs170/myFile2
Relative file path (cont.)
If the current directory is /home/cheung/cs170, then the
path for the indicated file (myFile2) is:
• myFile2
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)
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)
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:
• Microsoft Word
• Notepad
• We will learn to use gedit (GNU editor) in another webnote.
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 FILE-PATH
(cat is an abbreviation of the word catenate)
Print the content of a file out to the
terminal (cont.)
Example:
Catenating multiple files
• The cat command can catenate an arbitrary number of files
to the terminal
Example: catenate myFile1 and myFile2 to the terminal:
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)
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 " > FileName" to the command
• In other words:
• any UNIX command >
FileName
Redirecting the input and output to a file
(cont.)
• Example: redirecting the output of "ls" into a file:
Redirecting the input and output to a file
(cont.)
• Example: redirecting the output of cat myFile1 myFile2 to
a file (named myFile3)
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
Print a file to the printer
• Use this command to print a file to the default printer:
lpr File-Path
• 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
Print a file to the printer (cont.)
• Example:
lpr myFile1
will print the file named myFile1 in the current directory to
the printer
Delete a file
• The command (application) used to delete a file is:
rm FilePath
The word rm is an acronym for remove
All files with names matching the File-Path will be
removed
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/cs170)
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 !!!
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
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: [email protected] if you
need to recover a file using a backup copy.
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
Rename a file (cont.)
Important:
•
The new-File-Path file must not exist; otherwise, 2
things can happen:
1. 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)
2. If the new-File-Path exists and it is the name of a
directory, then the mv will move the file old-FilePath into the directory new-File-Path
Rename a file (cont.)
• Example:
mv myFile2 decl-of-indep
will rename the file named myFile2 to the new name declof-indep
Rename a file (cont.)
• Illustrated:
Notice that after the file is renamed, the content of the file
remains unchanged !
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 dirPath
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 !!!)
Move a file from one directory (folder) to
another directory (folder) (cont.)
• Illustrated:
mv command with nonexisting directory
mv command with existing
directory
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