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Linux Command – part2. Changing to a different directory. % cd Will take you to the parent % cd / This command will take you to the root directory (cygwin. Environment variables : . where the shell stores information that may be useful to the user’s shell session.

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Changing to a different directory
Changing to a different directory

  • % cd

    Will take you to the parent

  • % cd /

    This command will take you to the root directory (cygwin


Environment variables
Environment variables :

where the shell stores information that may be useful to the user’s shell session.

Examples of environment variables include:

  • HOSTNAME=localhost

  • SHELL=/bin/bash

  • HOME = your home

    To print the variable value, Type:

    echo $SHELL


Reading manual
Reading manual

  • man = manual

    $man

    With the man command, you can retrieve the information in the manual and display it as text output on your screen. To use the man command.

  • IF you want to read a manual for ls command, type:

    $ man ls

    To exit from the manual page, simply press Q


File permission
File permission

  • Keep users from accessing other users’ private files

  • To protect important system files

  • permissions bits

    rwxrwxrwx

    Owner’s| Group | Others

    r = read w = write x = execute


File permission cont
File permission (cont.)

For Files:

  • "Read" means to be able to open and view the file

  • "Write" means to overwrite or modify the file

  • "eXecute" means to run the file as a binary files are executable only if they are programs and shell scripts, not useful for data files.

    For Directories:

  • "Read" means to be able to view the contents of the directory

  • "Write" means to be able to create new files/directories or delete files/directories within the directory

  • "eXecute" means to be able to "Change Directory" (cd) into the directory = permission to access the directory.

  • How to view the permission for a file or directory?

    • $ ls -al


  • File permission cont1
    File permission (cont.)

    For Files:

    • "Read" means to be able to open and view the file

    • "Write" means to overwrite or modify the file

    • "eXecute" means to run the file as a binary files are executable only if they are programs and shell scripts, not useful for data files.

      For Directories:

    • "Read" means to be able to view the contents of the directory

    • "Write" means to be able to create new files/directories or delete files/directories within the directory

    • "eXecute" means to be able to "Change Directory" (cd) into the directory = permission to access the directory.

  • How to view the permission for a file or directory?

    • $ ls -al


  • File permission cont2
    File permission (cont.)

    • Only the owner of a file can change its permission.

    • How to set file permission?

      • Use the command chmod(change file mode bits).

    • chmod has two notations:

      • Numeric(octal) notation.

      • Symbolic notation.


    Change permission on a file numeric
    Change permission on a file - numeric

    • the file permissions aren't represented by characters. Instead, they are represented by a three-digit octal number.

    • 4 = read (r)2 = write (w)1 = execute (x)0 = no permission (-)

    If the permission is

    725

    4+2+1

    0+2+0

    4+0+1

    r-x

    rwx

    -w-


    Change permission on a file symbolic
    Change permission on a file – symbolic

    • Permissions are represented by characters rwx

    • This gives “who” the specified permissions for a given filename.

    • The “who” is a list of letters re going to be giving permissions to. These may be specified in any order.

    • +  add the selected permission.

    • -  remove the selected permission.


    Change permission on a file
    Change permission on a file

    For r, w, x octal value is 4,2,1 respectively

    owner(u) group(g) other(o) all(a)rwxrwxrwx

    owner(u) group(g) other(o) all(a)---------



    Using file matching metacharacters this matches any one of the characters between the brackets
    Using file-matching metacharactersThis matches any one of the characters between the brackets […]



    Echo command
    Echo command

    • Use echo command to display text or value of variable.

    • "Double Quotes“

      • literally print everything inside the single quote. Even the special variables such as $HOSTNAME will be print as $HOSTNAME

    • 'Single quotes‘

      •  display the real meaning of special variables

    • `Back quote`

      • Used with commands only.

      • To execute command


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