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7/28/2019 Practice 20th Nov
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SERVER DETAILS
NAME : bias20
OS : OEL 5.4
ip address: 172.16.12.222
port no: 5906
USER AND GROUP ADMINISTRATION :
To create new user in linux
#useradd newusername
#useradd user2
After creating user we have to create password for it and this is done by the command shown above
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#passwd username
It prompts for new unix password give it and press enter. The it asks for retype new unix password givw
it and press enter
By default the user list reside in /etc/passwd, so we can view the registered user by looking up at this
file
#cat /etc/passwd
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It will get all the user names,
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As we all know that by default all the users created will have their home directories in /home share. to
list onle home users the command is
#cat /etc/passwd | grep "/home"
Now we'll get all the user accounts which have their home share in /home
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To see only the list of users we have to modify the command to
#cat /etc/passwd | grep "/home" |cut -d: -f1
-d: means delimiter
-f1 means display first field of line i.e. username
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groupadd command is used to add a group in linux and the syntax is
#groupadd groupname
#groupadd group2
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/etc/group contains the list of groups created
#cat /etc/group
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useradd command is used to add new users to existing group or create a new group and then add user
The syntax is as follows:
#useradd -G {group-name} username
#useradd -G group1 user3
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we can see the group and its users by using grep command
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Addind a new user to multiple groups at a time
#useradd -G group1,group2 user4
capital (-G) option add user to a list of supplementary groups
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usermod is used to add an exting user to a group
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FILES AND DIRECTORY PERMISSIONS
Every file on your Linux system, including directories, is owned by a specific user and group.
Therefore, file permissions are defined separately for users, groups, and others.
User: The username of the person who owns the file. By default, the user who creates the file
will become its owner.
Group: The usergroup that owns the file. All users who belong into the group that owns the file
will have the same access permissions to the file. This is useful if, for example, you have a
project that requires a bunch of different users to be able to access certain files, while others
can't. In that case, you'll add all the users into the same group, make sure the required files areowned by that group, and set the file's group permissions accordingly.
Other: A user who isn't the owner of the file and doesn't belong in the same group the file does.In other words, if you set a permission for the "other" category, it will affect everyone else by
default. For this reason, people often talk about setting the "world" permission bit when they
mean setting the permissions for "other."
There are three types of access permissions on Linux: read, write, and execute. Thesepermissions are defined separately for the file's owner, group and all other users.
Read permission. On a regular file, the read permission bit means the file can be opened andread. On a directory, the read permission means you can list the contents of the directory.
Write permission. On a regular file, this means you can modify the file, aka write new data tothe file. In the case of a directory, the write permission means you can add, remove, and rename
files in the directory. This means that if a file has the write permission bit, you are allowed to
modify the file's contents, but you're allowed to rename or delete the file only if the permissions
of the file's directory allow you to do so.
Execute permission. In the case of a regular file, this means you can execute the file as aprogram or a shell script. On a directory, the execute permission (also called the "search bit")
allows you to access files in the directory and enter it, with the cd command, for example.However, note that although the execute bit lets you enter the directory, you're not allowed to list
its contents, unless you also have the read permissions to that directory.
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We can view the access permissions of a file by doing the long directory listing with thels -l
command
In the above we created a directory with name dir1 and in that we created a file name file1.
To see the permissions on that file we used the command ls -l
#ls -l
we got the output as:
total 4
-rw-r--r-- 1 root root 0 nov 20 21:20 file1
this output shows thatuser has read and write permission (rw-)
group has only read permission (r--)
others has only read permission (r--)
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We can set file permissions with the chmodcommand
Both the root user and the file's owner can set file permissions
chmod has two modes, symbolic and numeric
The symbolic mode is pretty easy to remember. First, we has to decide to which we has to set
permissions for the user (u), the group (g), others (o), or all of the three (a).
Then we either add a permission (+), remove it (-), or wipe out the previous permissions and add a new
one (=).
Next, we decide whether to set the read permission (r), write permission (w), or execute permission (x).
Last, we'll tell chmod which file's permissions you want to change
EXAMPLES
Wipe out all the permissions but add read permission for everybody
#chmod a=r file1
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Add execute permissions for group
# chmod g+x testfile
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Remove the execute permission from both the file's owner and group
# chmod ug-x file1
In the numeric mode, 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 (-)To get the permission bits we want, we add up the numbers accordingly.
For example,
the rwx permissions would be 4+2+1=7,
rx would be 4+1=5,
and rw would be 4+2=6.
Because we set separate permissions for the owner, group, and others, we'll need a three-digit number
representing the permissions of all these groups
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# chmod 755 file1