Linux Command – part2

1. Linux Command – part2

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

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. • 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-

10. 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.

11. 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)---------

12. Using file-matching metacharacters

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

14. Using file-matching metacharacters

15. 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