Linux Shell and Per Missions and Tar

8/3/2019 Linux Shell and Per Missions and Tar

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LINUX SHELL

• In Linux Os Shell is a special program that acceptsinstructions or commands in English, if it’s a validcommand and pass it to kernel. Shell is a commandlanguage interpreter that executes commands read

from the standard input device(keyboard) or from afile. Shell is not a part of system kernel but uses thesystem kernel to execute the programs, create files etc.

• To find the our current shell following command isused:-

#echo $ shell



Linux/Unix Shells



• Shell script defined as:

"Shell Script is series of command written in plain text file. Shell script is just like batch file

is MS-DOS but have more power than the MS-

DOS batch file."



Variable

• A variable is the label that has a value which

will be used to configure the shell.

# set / less

# set / env



File Permissions in LINUX

• Every folder or file in Linux has access

permission. There are three types of

permissions(what allowed to do with a file):-

• Read access – “r”

•

Write access – “w” • Execute access – “x”

• If access type is denied “-”



• Permissions are defined for three types of

users:-

1. The owner of the file2. The group that the owner belongs to

3. Other users

Thus linux file permissions are nine bits of information (3types by 3types of users), each

of them may have just one or two values:

allowed or denied.



File Permissions Notations

1. Textual representation like “-rwxr—r-”

It is used in linux long directory listings. Itconsists of 10 characters.The first character

shows the file type.Next 9 characters arepermissions consisting of three groups:owner,group,others.Each group consists of 3symbols: rwx , if some permission is deniedthen a dash “-” is used. E.g “-rwxr--r--”.

0123456789



• Symbol in the position 0 “-” is the type of

file.It is “d” if the item is a directory and “l” if

the item is a link.

• Symbols in the position 1 to 3 “rwx” are

permissions for the owner of the file.

• Symbols in the position 4 to 6 “r--” are

permissions for the group.

• Symbols in the position 7 to 9 “r--” are

permissions for the others.





2.Numeric (octal) Representation

• Digits involved are 0 to 7.

• Example “644”

Here first digit stands for the owner,second digit stands for the group,

third digit stands for right of others







How to set file permissions

To apply permission there is a command called

as “chmod”. e. g #chmod 755 testfile. This

would change the testfile permissions to

-rwxr-xr-x. It means that owner would have

full read, write and execute permissions,

group and others will have read and execute

permissions.



How to create a TAR file in Linux

• Tape Archive (TAR) files were used in Linux to

back up data to tape. They function similarly

to the ZIP file format in the windows

environment.

• Command is: #tar –cvf myfile.tar filename1

filename2 where myfile.tar is the name of the

TAR file.



To check the contents in a TAR file

• Command is:

#tar –tvf myfile.tar

The contents of the myfile.tarfile will be displayed.



To move a file in TAR folder

• Command is:

#tar –rvf myfile.tar yahoo



Extracting the files from a TAR file

• Command is:

#tar –xvf myfile.tar

the system would uncompress (untar) the

myfile.tar file in the current directory.



Creating a Tarred file that is

compressed with zip

• Command is:

#tar –czvf myfile.tar.gz myfile.tar

















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Linux Shell and Per Missions and Tar