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Tony Kombol. Linux Command Basics II. 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. su. Switch user. su. su userid su is short for s witch u ser

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Tony kombol

Tony Kombol

Linux Command Basics II

Man review
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


Switch user


  • 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!


  • 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 superuser

  • Fortunately, there is a program to give temporary access to the superuser's privileges:

    • su userid

    • If userid not specified, root is assumed


  • 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:

      [[email protected] me]$ su


      [[email protected] 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)

File access
File Access


What are permissions
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 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

What are permissions1
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

Files permissions
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 (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).

Files permissions1
Files Permissions

  • To see the permission settings for a file use the ls command with the –l option:

    [[email protected] me]$ ls -l some_file

  • Typical response:

    -rw-rw-r-- 1 me us 1097374 Sep 26 18:48 some_file

Files permissions2
Files Permissions

  • By looking at the returned permissions-rw-rw-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 and write this file

    • The file is owned by the group “us"

      • Members of the group “us" can also read and write this file

    • Everyone else can only read this file

Files permissions3
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


  • Change the file permissions (access):

    • chmod

      • modify file access rights

    • chown

      • change file ownership

    • chgrp

      • change a file's group ownership


  • 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

Chmod octal
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!

C hmod octal
chmod (octal)

  • By representing each of the three sets of permissions (owner, group, and other) as a single octal digit

    • Have a convenient way of expressing the permissions settings

  • Example: set some_file to have read and write permission for the owner, but keep the file private from others, enter the command:

    • [[email protected] me]$ chmod 600 some_file

Table of some settings
Table of some settings

Directory permissions
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)

C hmod symbolic mode
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 symbolis 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

C hmod symbolic mode1
chmod (symbolic mode)

  • Examples:

    • chmod u+w

      • Add the ability of the owner (user) to edit

        • Really to save it after it is changed

      • Does not change any other permissions

    • chmod a-x

      • Remove execute for everyone (all)

C hmod octal or symbolic
chmod: octal or symbolic?

  • Octal:

    • Can set all permissions with 3 characters

    • Easiest to change all permissions to an exact specification

  • Symbolic:

    • Can easily alter one permission for a reference

  • Know and use both effectively!


  • 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”:

    • [[email protected] me]$ suPassword:[[email protected] me]# chown you some_file[[email protected] me]# exit[[email protected] me]$


  • Notice that in order to change the owner of a file, you must have root authority!

    • To do this, our example:

      • employed the sucommand for a root shell

      • executed chown

      • typed exit to return to our previous shell

  • chown works the same way on directories as it does on files


  • Change the group ownership of a file or directory

  • Command format:

    • [[email protected] 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

Job control1
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


  • 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).

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

[[email protected] me]$ xload &[1] 1292

[[email protected] me]$ jobs[1]+ Running xload &

[[email protected] me]$ kill %1

[[email protected] me]$ xload &[2] 1293[1] Terminated xload

[[email protected] me]$ psPID TTY TIME CMD1280 pts/5 00:00:00 bash1293 pts/5 00:00:00 xload1294 pts/5 00:00:00 ps

[[email protected] me]$ kill 1293[2]+ Terminated xload


  • Flexible and powerful text search utility

  • Basic syntax:

    • grep search-item file.searched

  • Can get very sophisticated


    • can use regular expressions

    • can search multiple files


  • “|”

    • Takes the output of one program and uses it as input to another

    • E.g.:

      • du | less


  • “>”

    • Take the output and (re)place in a file

      • Create the file if it does not exist

      • Replace the file if it does exist

    • “>>”

      • Appends data to existing file

  • “<“

    • Take the input from a new source (file)

Reminder autocomplete and recall
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

Your own top 10 linux commands
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

More Top Linux Commands for Newbie