- UNIX philosophy: Everything is a file
- Plain file
- Directory
- Block device (disk, CD, DVD, tape)
- Character device (keyboard, mouse, modem)
- Socket link (internet)
- FIFO (pipes between processes)
- Symbolic link (aliases, shortcuts)
- File systems
- Root file system
- Mounted file systems
- df command
- Traditionally off /mnt and /media, but other places too , e.g., /fs, /Volumes, etc.
- Local and NFS
- mount command
- The result is a single tree
- Each node has a number of attributes:
- Name
- Type
- Size
- Owner
- Group (each file belongs to one group, even if the owner belongs to multiple groups)
- Permissions
- Number of links
- Access dates
- ls -l gives a full listing
> ls -l total 80 -rw-r--r--@ 1 tyfeng staff 11691 Jan 7 17:18 FileSystem.shtml -rw-r--r--@ 1 tyfeng staff 13155 Jan 7 17:11 Mastery.shtml drwxr-xr-x 10 tyfeng staff 320 Jan 7 17:11 MyCode drwxr-xr-x 4 tyfeng staff 128 Jan 7 17:10 Pictures -rw-r--r--@ 1 tyfeng staff 10859 Jan 7 17:17 ShellIntro.shtml
- stat gets information about a file
- Pathnames consist of file/directory names, separated by slashes
- Any character except / is allowed. Use ''s around weird file names.
- Extensions are not meaningful to UNIX, only to applications
- Special file names
- .file_name
- .
- ..
- Absolute and relative file names
- Absolute to the root (start with slash)
- Relative to the current working directory of the process (including . and ..)
- cat displays a file
cp copies files
mv renames files
rm deletes files
touch creates a file (if it dos not already exist), and sets last modification to now
chown changes the owner of a file (root only)
chgrp changes the group of a file (constraints)
du tells usage
- ls <directory> lists the files in the <directory>
(default is the current working directory)
- ls -F appends characters to show file type
- ls -a includes files starting with . (hidden files)
- ls has lots of useful options - read the man page
- ls -F appends characters to show file type
- cd changes the shell's current working directory
mkdir makes a directory
rmdir removes an empty directory
rm -r recursively removes files and directories
- Important system directories (common places - these vary between
UNICES)
- /bin - binaries of commands
- /sbin - binaries of system admin commands
- /usr - read-only data shared between machines
- /usr/bin - shared binaries
- /usr/sbin - shared system admin commands
- /usr/local/bin - shared binaries installed locally
- /usr/local/sbin - shared system admin binaries installed locally
- /dev - files corresponding to devices
- /etc - host specific configuration files
- /home - user accounts
- /lib - libraries of objects
- /tmp - temporary stuff, deleted periodically
- /var - changing data
- /proc - information about processes
- UNIX file systems are organized as directed acyclic graphs (DAGs).
Think of the names being on the edges, not the nodes.
- A node can have multiple names, "linked" from different parts of
the hierarchy.
- Hard links cannot be to directories, or across file systems
- Symbolic links are different, and provide a reference to another node in the hierarchy, for the system to follow.
- ln makes hard links
- Example: ln example.txt hard_link
- example.txt and hard_link point to the same data
- deleting example.txt does not affect hard_link.
- Example: ln -s example.txt soft_link
- soft_link points to example.txt
- deleting example.txt makes soft_link invalid.
- There are three groups with access rights:
- The user who owns the file
- Members of the group associated with the file
- Other users
- There are three types of access:
- Read access
- Write access
- Execute (file) or current directory (directories) access
- Symbolic encoding
- The mode has the form uuugggooo to encode user, group, and other access rights
- Each letter may be one of -, r, w, x.
- chmod changes permissions
- Numerical Encoding
- The 9 permission bits are represented by 3 octal digits
(each digit represents 3 bits):
- 001 represents execute access for others
- 002 represents write access for others
- 004 represents read access for others
- 010 represents execute access for group
- 020 represents write access for group
- 040 represents read access for group
- 100 represents execute access for user
- 200 represents write access for user
- 400 represents read access for user
- To generate a composite mode, add up the individual modes
- Example:
rw-r--r-- is a common mode for a regular file:
the user is allowed to read and write the file, and
everyone else is allowed to read the file (no secrets ;-).
- Corresponding numerical value:
0400 User read 0200 User write 0040 Group read 0004 Other read --------------- 0644
- Corresponding numerical value:
- chmod changes permissions
- The 9 permission bits are represented by 3 octal digits
(each digit represents 3 bits):
Exercises
- When logged in as the root user, how often would you do the command rm -rf /?
- Name the 7 types of files in UNIX.
- Describes the type(s) of files that are stored in
- /bin
- /sbin
- /usr
- /usr/bin
- /usr/sbin
- /usr/local/bin
- /usr/local/sbin
- /dev
- /etc
- /home
- /lib
- /tmp
- /var
- What access permissions does 641 give for a plain file?
- What access permissions does 754 give for a directory?
Exam Style Questions
- Explain the mechanism by which multiple physical disk partitions are viewed as a single file system in UNIX.
- What is the significance of a leading . on a file name?
- Which if the following are legal file names in UNIX?
- big_222_numbers
- .HiddenCode.c
- spaced out man
- /slash/
- $dollar$
- I'm a little Noddy Man, I always nod my head
- Describe how hard links give a file multiple names in UNIX.
- Describe how soft (symbolic) links give a file multiple names in UNIX.
- Describe what users can do what to the file MyProgram after
doing the command
chmod 510 MyProgram
- Describe what users can do what to the directory PartialSecrets
after doing the command
chmod 754 PartialSecrets
- Give a command that adds read and write permission to the file NoSecrets for the groups and others.