Linux Command Basics II

1. Tony Kombol Linux Command Basics II

2. man review… • man • on-line user manual • man command_you_want_info_on • type q to exit • examples: • for ls (list directory) • man ls • for cp (copy) • man cp

3. su Switch user

4. su • su userid • su is short for switch user • temporarily assume the id of another user • If no userid is supplied root is assumed • Hence the term superuser is sometimes assumed for su • Useful in cases when need to be the superuser for a small number of tasks • or any user!

5. su • It is often useful to become the superuser (root) to perform important system administration tasks • But as previously warned do not stay logged on as the root • Fortunately, there is a program to give temporary access another user's privileges: • su userid • If userid not specified, root is assumed

6. su • To become the superuser or root • Debian: • Type the su command • CentOS: • Type the su – command • The – is important, in CentOS it gives the privilages • Then prompted for root's password: [me@linuxbox me]$ su Password: [root@linuxbox me]# • After executing the su command, a new shell session as the superuser is started • To exit the superuser session • type exit • Returns to previous session (user)

7. File Access Permissions

8. What are Permissions? • The Unix operating system families are not only multitasking but are also a multi-user • What exactly does multi-user this mean? • More than one user can be operating on the computer at the same time • While the computer might only have one keyboard and monitor, it can still be used by more than one user • If the computer has serial ports users may access the computer via them • If a computer is attached to a network, or the Internet, remote users can log in via telnet or ssh (secure shell) and operate the computer

9. What are Permissions? • To make multiple access practical, a method had to be devised to protect the users from each other • Actions of one user should not crash the computer • One user should not be able interfere with the files belonging to another user

10. Files Permissions • Unix and Linux use the same permissions scheme • Each file and directory is assigned access rights for: • Owner of the file • By default the userid that created the file • Can be re-assigned • Members of a group of related users • By default the owner • Everybody else • e.g. the rest of the world • Rights can be assigned to: • read a file (look at it) • write a file (change it) • execute a file (run the file as a program).

11. Files Permissions • To see the permission settings for a file use the ls command with the –l option: [me@linuxbox me]$ ls -l some_file • Typical response: -rwxrw-r-- 1 me us 1097374 Sep 26 18:48 some_file

12. Files Permissions • By looking at the returned permissions-rwxrw-r-- 1 meus 1097374 Sep 26 18:48 some_file a lot can be determined from examining the results of this command: • It is a normal file • Signified by the “-” on the left • The file "some_file" is owned by user "me" • User "me" has the right to read, write & execute (run) this file • The file is owned by the group “us" • Members of the group “us" can read and write this file • Everyone else can only read this file

13. Files Permissions • To interpret the first portion (10 characters) of the listing: • First character indicates the file type • -: normal file • d: directory • l: link • Followed by three sets of three characters • Owner • First three • Group • Second three • Everybody else (world) • Last three • Each set conveys permissions for • Reading • First character • Writing • Second character • Execution • Last character

14. Permissions • Change the file permissions (access): • chmod • modify file access rights • chown • change file ownership • chgrp • change a file's group ownership

15. chmod • Change the permissions of a file or directory • Specify the desired permission settings and the file or files to be modified • Two ways to specify the permissions • Octal • Symbolic

16. chmod (octal) • Permission settings can be thought of as a series of bits for each grouping • First bit for read • Second for write • Third for execute • Example: • rwx rwx rwx = 111 111 111 • rw- rw- r-- = 110 110 100 • rwx --- --- = 111 000 000 • and so on... • Therefore for each group of 3 bits: • rwx = 111 in binary = 7 • rw- = 110 in binary = 6 • r-x = 101 in binary = 5 • r-- = 100 in binary = 4 • Etc. • Octal is perfect for noting permissions!

17. chmod (octal) • By representing each of the three sets of permissions (owner, group, and other) as a single octal digit • Convenient way of expressing the permissions • Example: • set some_file to have read and write permission for the owner, but keep the file private from others, enter the command: • [me@linuxbox me]$ chmod 600 some_file

18. Table of some settings

19. Directory Permissions • chmod command can also be used to control the access permissions for directories • Permissions scheme for directories similar to files • R – view directory contents • W - create or delete files in the directory • X – Allow users to change into directory (cd dir)

20. some useful settings for directories

21. chmod (symbolic mode) • chmod [ugoa] [+-=] [rwx] fname • The first symbol is the reference (who) to change: • u: user (owner) • g: group • o: others (world) • a: all • Multiple references may be specified… • The second symbol is to add, remove or set a permission • +: add the permission • -: remove the permission • =: make it the exact permission • The last symbol is the permission to change • r: is to read • w: is to write • x: is to execute • Multiple permissions may be specified

22. chmod (symbolic mode) • Examples: • chmod u+w change.sh • Add the ability of the owner (user) to edit (w) change.sh • Really to save it after it is changed • Does not change any other permissions • chmod a-x xyz.sh • Remove execute for everyone (all)

23. chmod: octal or symbolic? • Octal: • Can set all permissions with 3 digits • Easiest to change all permissions to an exact specification • Symbolic: • Can easily alter only one permission • Know and use both effectively!

24. chown • Change the owner of a file • File creator is the original owner by default • Only root can chown • Example: Change the owner of some_file from "me" to "you”: • [me@linuxbox me]$ suPassword:[root@linuxbox me]# chown you some_file[root@linuxbox me]# exit[me@linuxbox me]$

25. chown • Remember: in order to change the owner of a file, you must have root authority! • To do this, our example: • employed the su command for a root shell • executed chown • exited to return to original shell • chown works the same way on directories as it does on files

26. chgrp • Change the group ownership of a file or directory • Command format: • [me@linuxbox me]$ chgrp new_group some_file • In the example above • Group ownership of some_file changed from its previous group to "new_group" • Must be the owner of the file or directory to perform a chgrp

27. Job control

28. Job Control • Several commands available to control processes • The most commonly used: • kill • sends a signal to one or more processes • usually to "kill" a process

29. kill • Suppose a program becomes unresponsive • How to get rid of it? • Use the kill command • Let's try this out on xload: • First, identify the process to kill • Use either jobs or ps to do this • jobs will return a job number • ps returns a process id (PID).

30. Start xload and return control to terminal Use jobs to see who is running  [1] is the id number Kill id 1 Start xload again Use p to see the procsss status  1293 is xloads process id Kill process1293 [me@linuxbox me]$ xload &[1] 1292 [me@linuxbox me]$ jobs[1]+ Running xload & [me@linuxbox me]$ kill %1 [me@linuxbox me]$ xload &[2] 1293[1] Terminated xload [me@linuxbox me]$ psPID TTY TIME CMD1280 pts/5 00:00:00 bash1293 pts/5 00:00:00 xload1294 pts/5 00:00:00 ps [me@linuxbox me]$ kill 1293[2]+ Terminated xload

31. Other

32. grep • Flexible and powerful text search utility • Basic syntax: • grep search-item file_searched • Can get very sophisticated • http://unixhelp.ed.ac.uk/CGI/man-cgi?grep • can use regular expressions • can search multiple files

33. Redirection • “>” • Output of a program goes to std out by default • Usually the monitor • > sends the output to a file instead • Creates the file if it does not exist • Replaces the file contents if it does exist • “>>” • Appends data to an existing file • “<“ • Data is read from std in • Usually the keyboard • Reads the input from a different source (file)

34. Pipes • “|” • Takes the output of one program and uses it as input to another • E.g.: • du | less

35. Reminder: Autocomplete and Recall • Autocomplete • Tab key • Start the command or name • Hit Tab • If what you typed is unique up to that point bash will complete the rest of the typing! • If what you typed is not unique it will give a gentle beep • Hit Tab again to get a list of matches • Recall • The up arrow will recall previous commands • The down arrow recalls later commands

36. Wrap-up

37. Your own top 10 Linux commands • When commands are entered on the terminal they are kept in the history library • List your top 10 by with the history command: • History by itself gives the last ~500 commands • Use pipes and filters to get the reduced data history | awk '{print $2}' | awk 'BEGIN {FS="|"}{print $1}' | sort | uniq -c | sort -n | tail | sort -nr

38. More Top Linux Commands for Newbie