Linux Questions

How do you get last 15 lines from a file?

tail -15 error_log.txt


find vs grep

1. find searches for files and directories that match a user specified set of criteria. grep searches the contents of files for patterns that match those provided by the user.

2. find scours your drive looking for a certain series of letters -- like a file name or something. Like this:

Code:

find ~/documents/ -iname "*resume*"

That would look through a folder called "documents" for anything that included the word "resume" in the filename.

grep sifts through text output -- like from the context of a file -- for a certain string of characters. So

Code:

cat /proc/cpuinfo | grep model

should spit out a single line that shows the model name of your processor -- because the cat command dumps the contents of the /proc/cpuinfo file, and grep spits out only the line that contains the word "model".

So use find to look for a file, use grep to sift through the contents of a file or output. Of course, they both have other uses too.


Howto: Linux Add User To Group

You need to the useradd command to add new users to existing group (or create a new group and then add user). If group does not exist, create it. The syntax is as follows:
useradd -G {group-name} username
In this example, create a new user called vivek and add it to group called developers. First login as a root user (make sure group developers exists), enter:
# grep developers /etc/group
Output:

developers:x:1124:

If you do not see any output then you need to add group developers using groupadd command:
# groupadd developers
Next, add a user called vivek to group developers:
# useradd -G developers vivek
Setup password for user vivek:
# passwd vivek
Ensure that user added properly to group developers:
# id vivekOutput:

uid=1122(vivek) gid=1125(vivek) groups=1125(vivek),1124(developers)

Please note that capital G (-G) option add user to a list of supplementary groups. Each group is separated from the next by a comma, with no intervening whitespace. For example, add user jerry to groups admins, ftp, www, and developers, enter:
# useradd -G admins,ftp,www,developers jerry
useradd example - Add a new user to primary group

To add a user tony to group developers use following command:
# useradd -g developers tony
# id tony
Sample outputs:

uid=1123(tony) gid=1124(developers) groups=1124(developers)

Please note that small -g option add user to initial login group (primary group). The group name must exist. A group number must refer to an already existing group.
usermod example - Add a existing user to existing group

Add existing user tony to ftp supplementary/secondary group with usermod command using -a option ~ i.e. add the user to the supplemental group(s). Use only with -G option :
# usermod -a -G ftp tony
Change existing user tony primary group to www:
# usermod -g www tony

Compress & Un compress tar file

2. Understanding files and folders

Linux is made with one thought in mind: Everything is a file.

A blank piece of paper is called a file in the world of computers. You can use this piece of paper to write a text or make a drawing. Your text or drawing is called information. A computer file is another way of storing your information.

If you make many drawings then you will eventually want to sort them in different piles or make some other system that allows you to easily locate a given drawing. Computers use folders to sort your files in a hieratic system.

A file is an element of data storage in a file system (file systems manual page). Files are usually stored on harddrives, cdroms and other media, but may also be information stored in RAM or links to devices.

To organize our files into a system we use folders. The lowest possible folder is root / where you will find the user homes called /home/.

/
/home/
/home/mom/
/home/dad/

Behind every configurable option there is a simple human-readable text file you can hand-edit to suit your needs. These days most programs come with nice GUI (graphical user interface) like Mandrakes Control Center and Suses YAST that can smoothly guide you through most configuration. Those who choose can gain full control of their system by manually adjusting the configuration files from foo=yes to foo=no in an editor.

Almost everything you do on a computer involves one or more files stored locally or on a network.

Your filesystems lowest folder root / contains the following folders:
/bin Essential user command binaries (for use by all users)
/boot Static files of the boot loader, only used at system startup
/dev Device files, links to your hardware devices like /dev/sound, /dev/input/js0 (joystick)
/etc Host-specific system configuration
/home User home directories. This is where you save your personal files
/lib Essential shared libraries and kernel modules
/mnt Mount point for a temporarily mounted filesystem like /mnt/cdrom
/opt Add-on application software packages
/usr /usr is the second major section of the filesystem. /usr is shareable, read-only data. That means that /usr should be shareable between various FHS-compliant hosts and must not be written to. Any information that is host-specific or varies with time is stored elsewhere.
/var /var contains variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files.
/proc System information stored in memory mirrored as files.

The only folder a normal user needs to use is /home/you/ - this is where you will be keeping all your documents.

/home/elvis/Documents
/home/elvis/Music
/home/elvis/Music/60s

Files are case sensitive, "myfile" and "MyFile" are two different files.

For more details, check out:
3. Understanding users and permissions

Linux is based on the idea that everyone using a system has their own username and password.

Every file belongs to a user and a group, and has a set of given attributes (read, write and executable) for users, groups and all (everybody).

A file or folder can have permissions that only allows the user it belongs to to read and write to it, allowing the group it belongs to to read it and at the same time all other users can't even read the file.
4. Who and what is root

Linux has one special user called root (this is the user name). Root is the "system administrator" and has access to all files and folders. This special user has the right to do anything.

You should never log on as this user unless you actually need to do something that requires it!

Use su - to temporary become root and do the things you need, again: never log into your sytem as root!

Root is only for system maintenance, this is not a regular user (LindowsOS don't have any user management at all and uses root for everything, this is a very bad idea!).

You can execute a command as root with:

su -c 'command done as root'

Gentoo Linux: Note that on Gentoo Linux only users that are member of the wheel group are allowed to su to root.
5. Opening a command shell / terminal

To learn Linux, you need to learn the shell command line in a terminal emulator.

In KDE: K -> System -> Konsoll to get a command shell)

Pressing CTRL-ALT-F1 to CTRL-ALT-F6 gives you the console command shell windows, while CTRL-ALT-F7 gives you XFree86 (the graphical interface).

xterm (manual page) is the standard XFree console installed on all boxes, run it with xterm (press ALT F2 in KDE and Gnome to run commands).

Terminals you probably have installed:

* xterm http://dickey.his.com/xterm/
* konsole (KDEs terminal)
* gnome-terminal (Gnomes terminal)

Non-standard terminals should install:

* rxvt http://www.rxvt.org/
* aterm http://aterm.sourceforge.net

6. Your first Linux commands

Now you should have managed to open a terminal shell and are ready to try your first Linux commands. Simply ask the computer to do the tasks you want it to using it's language and press the enter key (the big one with an arrow). You can add a & after the command to make it run in the background (your terminal will be available while the job is done). It can be practical to do things like moving big divx movies as a background process: cp movie.avi /pub &. Jobs - the basics of job control
6.1. ls - short for list

ls lists the files in the current working folder. This is probably the first command to try out. It as a number of options described on the ls manpage.

Examples:

ls

ls -al --color=yes
6.2. pwd - print name of current/working directory

pwd prints the fully resolved name of the current (working) directory. pwd manpage.
6.3. cd - Change directory

cd stands for change (working) directory and that's what it does. The folder below you (unless you are in /, where there is no lower directory) is called "..".

To go one folder down:

cd ..

Change into the folder Documents in your current working directory:

cd Documents

Change into a folder somewhere else:

cd /pub/video

The / in front of pub means that the folder pub is located in the / (lowest folder).
7. The basic commands
7.1. chmod - Make a file executable

To make a file executable and runnable by any user:

chmod a+x myfile

Refer to the chmod manual page for more information.
7.2. df - view filesystem disk space usage

df -h

Filesystem Size Used Avail Use% Mounted on
/dev/hda3 73G 67G 2.2G 97% /
tmpfs 2.0M 24K 2.0M 2% /mnt/.init.d
tmpfs 252M 0 252M 0% /dev/shm

The flags: -h, --human-readable Appends a size letter such as M for megabytes to each size.

df manpage
7.3. du - View the space used by files and folders

Use du (Disk Usage) to view how much space files and folders occupy. Read the du manual page for flags and usage.

du is a part of fileutils.

Example du usage:

du -sh Documents/
409M Documents

7.4. mkdir - makes folders

Folders are created with the command mkdir:

mkdir folder

To make a long path, use mkdir -p :

mkdir -p /use/one/command/to/make/a/long/path/

Like most programs mkdir supports -v (verbose). Practical when used in scripts.

You can make multiple folders in bash and other shells with {folder1,folder2} :

mkdir /usr/local/src/bash/{old,new,dist,bugs}

mkdir manual page

The command rmdir removes folders.
7.5. passwd - changes your login password

To change your password in Linux, type:

passwd

The root user can change the password of any user by running passwd with the user name as argument:

passwd jonny

will change jonnys password. Running passwd without arguments as root changes the root password.

If you need to add several new users and give them password you can use a handy program like Another Password Generator to generate a large set of "random" passwords.
7.5.1. KDE

From KDE you can change your password by going:

* K -> Settings -> Change Password
* K -> Settings -> Control Center -> System Administration -> User Account

7.6. rm - delete files and folders, short for remove

Files are deleted with the command rm:

rm /home/you/youfile.txt

To delete folders, use rm together with -f (Do not prompt for confirmation) and -r (Recursively remove directory trees):

rm -rf /home/you/foo/

Like most programs rm supports -v (verbose).

rm manual page
7.7. ln - make symbolic links

A symbolic link is a "file" pointing to another file.

To make a symbolic link :

ln /original/file /new/link

This makes /original/file and /new/link the same file - edit one and the other will change. The file will not be gone until both /original/file and /new/link are deleted.

You can only do this with files. For folders, you must make a "soft" link.

To make a soft symbolic link :

ln -s /original/file /new/link

Example:

ln -s /usr/src/linux-2.4.20 /usr/src/linux

Note that -s makes an "empty" file pointing to the original file/folder. So if you delete the folder a symlink points to, you will be stuck with a dead symlink (just rm it).

ln manual page
7.8. tar archiving utility - tar.bz2 and tar.gz

tar (manual page) is a very handle little program to store files and folders in archives, originally made for tapestreamer backups. Tar is usually used together with gzip (manual page) or bzip2 (manual page), comprepssion programs that make your .tar archive a much smaller .tar.gz or .tar.bz2 archive.

kde

You can use the program ark (K -> Utilities -> Ark) to handle archives in KDE. Konqueror treats file archives like normal folders, simply click on the archive to open it. The archive becomes a virtual folder that can be used to open, add or remove files just as if you were working with a normal folder.
7.8.1. tar files (.tar.gz)

To untar files:

tar xvzf file.tar.gz

To tar files:

tar cvzf file.tar.gz filedir1 filedir2 filedir2...

Note: A .tgz file is the same as a .tar.gz file. Both are also often refered to as tarballs.

The flags: z is for gzip, v is for verbose, c is for create, x is for extract, f is for file (default is to use a tape device).
7.8.2. bzip2 files (.tar.bz2)

To unpack files:

tar xjvf file.tar.bz2

To pack files:

tar cvjf file.tar.bz2 filedir1 filedir2 filedir2...

The flags: Same as above, but with j for for bzip2

You can also use bunzip2 file.tar.bz2 , will turn it into a tar.

For older versions of tar, try tar -xjvf or -xYvf or -xkvf to unpack.There's a few other options it could be, they couldn't decide which switch to use for bzip2 for a while.

How to untar an entire directory full or archives?

.tar:

for i in `ls *.tar`; do tar xvf $i; done

.tar.gz: for i in `ls *.tar.gz`; do tar xvfz $i; done

.tar.bz2: for i in `ls *.tar.bz2`; do tar xvfj $i; done