Directory commands

 

• ls [option] [(filename $\mid$ directory)...]: For each file or directory argument, ls lists the contents of the specified one. If no file are specified, contents of current working directory are listed. A typical ls output will list the files and directories only by their names. Most used option 'l' will list all files and directories in a single line with permissions, number of links, owner, size, last update date and name.

    <mendelson:~/tmp> ls -l
    total 4
    lrwxrwxrwx   1 selcuk          4 Oct 14 17:55 dnm -> hebe
    -rw-r--r--   1 selcuk         24 Oct 14 17:43 hebe
    -rw-r--r--   2 selcuk         69 Oct 14 17:43 ordek
    -rw-r--r--   2 selcuk         69 Oct 14 17:43 ordek
    drwxr-xr-x   2 selcuk        512 Oct 14 17:44 subdir
    <mendelson:~/tmp>
  

  In the example above, there are three files, one directory and one symbolic link. The leftmost bit indicates what kind of a file it is. Possible kinds are listed below.

  
  		 d 		 Directory
  		 l 		 Soft link (Not a real file. Used as a shortcut)
  		 c 		 Character-type special file (Device files)
  		 b 		 Block-type special file (Device files)
  		 p 		 FIFO file (pipe)
  		 s 		 Socket file (Used for communication between processes)
  

• chmod mode (filename $\mid$ directory): The mode of each named file or directory is changed according to mode, which may be absolute or symbolic.
  • Absolute mode can be calculated simply by grouping the permissions to three octal numbers (modulo 8). If a permission is given, the corresponding bit is one and if it is not given, corresponding bit is zero. Note that this command can be used for both files and directories.
    

    Example: You want to give read, write ,execute permissions to yourself, and read, execute permissions to group and others for the directory tmp. The corresponding permission bits are marked as one, and others are marked as zero. Starting from left, group three bits and change them to octal numbers by sequence. In our example, leftmost number is 7, middle number is 5 and rightmost number is 5.

        owner     group     other
       r  w  x   r  w  x   r  w  x         r:read permission
       1  1  1   1  0  1   1  0  1         w:write permission
       \     /   \     /   \     /         x:execute permission
         -|-       -|-       -|-  
          7         5         5
    

    Once you calculate the number you should give the command "chmod 755 tmp". If you ls the directory, the output will look as follows.

    mendelson:~> ls -ld tmp
    drwxr-xr-x   2 selcuk        512 Oct 14 17:26 tmp
    mendelson:~>
    

    If you ignore the leftmost letter 'd', remaining nine characters are the corresponding permission bits. If there is a '-' for a bit, this means that the corresponding user does not have that permission. In out example, group and other does not have the second permission (write permission).
    
  • using chmod with symbolic permissions is easier in some situations. The symbolic permissions have the following syntax:
    [who] operator [permissions]
    
    • who zero or more of the characters u, g, o and a specifying whose permissions are to be changed or assigned:
      u user's permissions g group's permissions o others' permissions a all permissions (user, group, and other)
      If who is omitted, the default is a.
      
    • operator either +, - or = stating how the permissions will be changed.
      + Add permissions to the current. - Remove permissions from the current = assign permissions absolutely. (This has the same effect with absolute mode parameter explained in the above item.)
      
    • permission any compatible combination of the following letters:
      r read permission w write permission x execute permission l mandatory locking s user or group set-ID t sticky bit u,g,o indicate that permission is to be taken from the current user, group or other mode respectively.
      
      For example, to add read and write permissions for group of the file example.ceng111, you have to give the following command "chmod g+rw example.ceng111". The resulting mode bits of the file will be as follows.
      

      <mendelson:~> ls -l ceng111
      -rwx--x--x   1 selcuk        716 Oct  6 11:01 ceng111*
      <mendelson:~> chmod g+rw ceng111
      <mendelson:~> ls -l ceng111
      -rwxrwx--x   1 selcuk        716 Oct  6 11:01 ceng111*
      

      As you can see from the example, the permissions of owner and others did not change. Also the existing permissions of group (x) was not deleted. The read and write permissions were added on.
      
• cd [directory]: cd changes the current directory to directory. Directory can be represented in three ways. Either as a absolute path or relative path or relative path to user directory. If the directory is not specified, then you will end up in your home directory.
  Example: Current directory is /home1/selcuk/tmp/subdir/second/dnm, and user directory is /home1/selcuk. All three commands below will end up in the same directory although different representations are used.

  <mendelson:~/tmp/subdir/second/dnm> cd /home1/selcuk/tmp 
  <mendelson:~/tmp/subdir/second/dnm> cd ../../..
  <mendelson:~/tmp/subdir/second/dnm> cd ~/tmp
  

  Note that there exists a '~' character instead of /home1/selcuk in the prompt.
• mkdir directory: mkdir creates specified directories with the mode determined by the umask setting. The dirname can be given as one of the three path styles.
• rmdir directory: rmdir removes the specified directory if it is empty. (Does not contain files or subdirectories in it.)
• pwd : pwd prints the path of the working (current) directory starting from the root directory.