Linux Files

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Linux Files
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File Characteristics

• To the average user, a file is a sequence of
  information bytes, stored together and named as a
  collective entity. A file can be an executable
  program, a document, a database, a shell script,
  or any other collection of bytes that has
  meaning. In fact, virtually everything in Linux
  is seen by the system as a file, including the
  directories and devices.

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File Characteristics

• Filenames in Linux can range up to 255 characters
  and may or may not be terminated with an
  extension. Filenames are case sensitive, like
  nearly everything else in Linux. This means that
  the names File.txt, FILE.txt, file.txt, and
  FiLe.txt would all refer to different files.

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Determining a Linux Files Type

• You can use commands like ls and file to
  determine the type.
• The file command was created to allow a user to
  determine a files type and is the best tool for
  this purpose. The format for the file command is
• file options -f namefile -m magicfiles
  file
• The ls command is one of the most often used
  Linux commands . ls provides information about a
  file, including its type. The name ls means
  listing because the commands lists all of the
  files in a directory. The syntax for ls is
  simple
• ls options file or directory name

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The file command

• The ls command doesnt differentiate between the
  various normal file types, the file command
  does. The file command is supported by a file
  named magic (/usr/share/magic by default on a Red
  Hat system) which contains information on all the
  file types that the file command knows about. The
  f option reads in a file which contains names of
  files whose type is to be determined. You can
  specify a different magic file with the m option
  on the command line. The file command will use
  this magic file to determine, to the best of its
  knowledge, what type of file youve specified.
  The file commands output will usually include
  one of the following key words text, executable,
  or data.

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File Types Defined
File Types Recognized by ls
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File-Handling Commands
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chmod

• SYNTAX
• chmod options mode(s) filename(s)
• chmod options octal_mode(s) filename(s)
• The chmod command is used to change the access
  mode of files. Only the owner of the file or the
  superuser may alter its access. There are two
  methods for the expressing the mode you wish to
  assign. The first is the symbolic method, wherein
  you specify letters representing the mode. This
  requires that you specify the following
  information.

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chmod

• Who is affected
• u User who owns the file
• g Group (only users in files group)
• o Other users
• a All (default)
• What operation
• Add permission
• - Remove permission
• Set permission, overwriting old
  permissions

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chmod

• What kind of permission
• r Read
• w Write
• x Execute
• s User or group ID is temporarily replaced
  with that of the file
• t Set sticky bit keep executable in memory
  after exit

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chmod

• For example, ugx would add execute privileges
  for the user and members of the group, and orw
  would allow other users not in the specified
  group to read and write the file.

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chmod (octal mode)

• Some administrators prefer the octal method,
  which uses a sequence of three numbers to
  represent the permissions for the user, group,
  and others. The new permissions completely
  override the previous assignment. Three digits
  are computed, representing the user, group, and
  others, respectively.

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chmod (octal mode)

• To compute them, you add up the integers
  corresponding to the permissions you wish to
  grant each level. The result is a three-digit
  number in which the first number represents the
  User permissions, the second the Group
  permissions, and the third the Other permissions.
  The values assigned to each permission are as
  follows
• Execute
• Write
• 4 Read

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chmod (octal mode)

• Thus, read and write permissions would assign a 6
  (24). Read, write, and execute would assign a 7
  (124). Using this method, 755 would grant the
  user read, write, and execute privileges, and
  both group members and all others would have read
  and execute.

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chown

• SYNTAX
• chown options newowner filename(s)
• chown options newowner.newgroup filename(s)
• The chown command changes the owner of the
  specified file or files to the owner listed as an
  argument. This command can also be used to change
  both the owner and the group settings on the
  specified file. To do this, append a period
  followed by the new group to the owner name.

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chgrp

• SYNTAX
• chgrp options newgroup filename
• The chgrp command is used to change only the
  group setting for the file. You must own the file
  or be the superuser to use this command. The new
  group may be specified by the group name or ID.