Linux Command – part2

1. Linux Command – part2

2. Changing to a different directory • % cd Will take you to the parent directory • % cd / This command will take you to the root directory

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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.

9. Change permission on a file - numeric • The file permissions aren't represented by characters. Instead, they are represented by a three-digit octal number. The first digit specifies the permission of the Owner The second digit specifies the permission of the Groups The third digit specifies the permission of the Others

10. Change permission on a file - numeric • 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-

11. 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.

12. 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)---------

13. Change permission on a file - Examples • Using numeric notation, change the permission of a file called S.txt so that the owner can read, write and execute it where the group can write and execute and others can only read. The first digit will specifies the owner’s permission : 4 ( read ) + 2 (write ) + 1 ( execute ) = 7. The second digit will specifies the group’s permission : 0 (no read ) + 2 (write ) + 1 ( execute ) = 3. The third digit will specifies the others’ permission : 4 (read ) + 0 (no write ) + 0 (no execute ) = 4 So the command will be : chomd 734 S.txt

14. Change permission on a file - Examples • Using symbolic notation, remove write permission from the owner and groups. • chmod gu-w

15. Using file-matching metacharacters

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

17. Using file-matching metacharacters

18. Using file-matching metacharacters – Examples • Write a command to list any file that begins with B. ls B* * means : any number of characters. • Write a command to list any file begins with d and ends with f ls d*f Write a command to list any file begins with a or b , followed by 2 characters. ls [ab]??

19. 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