- 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 72 -rw------- 1 geoff user 490 Feb 10 2004 CSCCodes drwx------ 5 geoff user 4096 Mar 25 2008 CVSLab drwx------ 2 geoff user 4096 Apr 22 2004 LabTasks drwx------ 2 geoff user 4096 Apr 7 2008 MyCode.C drwxr-xr-x 2 geoff user 4096 Jan 21 10:03 MyCode.UNIX drwx------ 2 geoff user 4096 Apr 27 2004 RPC drwx------ 2 geoff user 4096 Feb 6 2007 SampleCode -rw------- 1 geoff user 12552 Jan 18 2005 Schulz.tgz drwx------ 2 geoff user 4096 Apr 22 2008 SocketExample drwxr-xr-x 2 geoff user 4096 Feb 26 2004 StreamEditor drwxr-xr-x 2 geoff user 4096 Apr 10 2008 ThreadFib drwx------ 2 geoff user 4096 Apr 30 2007 ThreadFibWithSignals -rwx------ 1 geoff user 5119 Jan 25 2007 Ticker -rw-r--r-- 1 geoff user 418 Feb 3 2006 Ticker.c
- stat gets information about a file
- Pathnames consist of file/directory names, separated by slashes
(as opposed to sloshes)
- 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 (does not "move" them)
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
pushd pushes the shell's current working directory
popd pops 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 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
- Hard links don't work on mounted file systems (or at least they did not show multiple links on the local side, only on the remote side).
- Symbolic links are different, and provide a reference to another node in the hierarchy, for the system to follow.
- ln makes hard links
ln -s makes symbolic links
stat and readlink dereference symbolic links
- There are three groups with access rights:
- The user who owns the file
- Members of the group associated with the file
- Other users
- If the process has the EUID of the file owner, the first group of permissions is used. If the process has the EGID of the file group, the second group of permissions is used. If neither of the former apply, the third group of permissions is used.
- 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):
- Default permissions
- The default permissions for new plain files are rw-rw-rw-, and the default permissions for new directories are rwxrwxrwx.
- Each process maintains a file mode umask that determines what access rights are not granted for newly created files and directories.
- The umask is inherited by new processes (i.e., your files will be created with rights based on the umask of your shell)
- The umask contains 9 bits, corresponding to the rwxrwxrwx access rights, and bits set in the mask are always cleared
- umask sets the umask of your shell