chmod linux command

chmod command is used to change the file access permission.


Simple example:

1) chmod 644 file1.txt

This gives the file read/write by the owner and only read by everyone else (-rw-r--r--).

2) chmod 755 file2.txt

This gives the file read/write/execute by the owner and read/execute by others (-rwxr-xr-x).




Syntax:
chmod [permission] file name


options:


-c, --changes    : report only when a change is made

-f,  --quiet       : suppress most error messages

      --help        : display this help and exit
-R,  --recursive : change files and directories recursively

-v,  --verbose   : output a diagnostic for every file processed




Permissions:
u - User who owns the file.
g - Group that owns the file.
o - Other.
a - All.

r - Read the file.
w - Write or edit the file.
x - Execute or run the file as a program.





Numeric Permissions:
400 read by owner
040 read by group
004 read by anybody (other)


200 write by owner
020 write by group
002 write by anybody


100 execute by owner
010 execute by group
001 execute by anybody

chgrp linux command

The chgrp command is used to change the group that has access to files and directories. All files in linux belong to an owner, and a group. The owner is set by the chown command, and the group by the chgrp command.

Syntax:

chgrp newgroup filename(s) [-options]

Options :

       -c, --changes

              like verbose but report only when a change is made

       --dereference

              affect the referent of each symbolic link, rather than the symbolic link itself (this is the default)

       -h, --no-dereference

              affect each symbolic link instead of any referenced file (useful only on systems that can change the ownership of a symlink)

       --no-preserve-root do not treat '/' specially (the default)

       --preserve-root

              fail to operate recursively on '/'

       -f, --silent, --quiet

              suppress most error messages

       --reference=RFILE

              use RFILE's group rather than the specifying GROUP value

       -R, --recursive

              operate on files and directories recursively

       -v, --verbose

              output a diagnostic for every file processed

       The  following  options modify how a hierarchy is traversed when the -R option is also specified.  If more than one is specified, only the final

       one takes effect.

       -H     if a command line argument is a symbolic link to a directory, traverse it

       -L     traverse every symbolic link to a directory encountered

       -P     do not traverse any symbolic links (default)

       --help display this help and exit

       --version

              output version information and exit

Examples:

       1) chgrp tech /file.txt

              Change the group of /file.txt to "tech".

       2) chgrp -hR tech /file.txt

              Change the group of /file.txt and subfiles to "tech".


      

cat linux command

The cat command's primary function is to concatenate, or join, files, but you can also use it to display a file on the screen. The cat command takes one or more files that you specify, smashes them together, and then sends the resulting file to the standard output device (without changing any of the original files). The standard output device is your screen, but by using the greater-than (>) operator, you can redirect cat's output to another file. 


Syntax :


cat [option] [file name]


Options :
      -A, --show-all
              equivalent to -vET


       -b, --number-nonblank
              number nonblank output lines


       -e     equivalent to -vE


       -E, --show-ends
              display $ at end of each line


       -n, --number
              number all output lines


       -s, --squeeze-blank
              never more than one single blank line


       -t     equivalent to -vT


       -T, --show-tabs
              display TAB characters as ^I


       -u     (ignored)


       -v, --show-nonprinting
              use ^ and M- notation, except for LFD and TAB


       --help display this help and exit


       --version
              output version information and exit



Examples:

cat test.txt

Display the test.txt file on the screen.

cat test1.txt test2.txt

Join the files test1.txt and test2.txt and then displayt he result on the screen.

cat test1.txt test2.txt > test3.txt

Join the files test1.txt and test2.txt and save them in the file test3.txt.

what is port scan

A port scan is a series of messages sent by someone attempting to break into a computer to learn which computer network services. Essentially, a port scan consists of sending a message to each port, one at a time. The kind of response received indicates whether the port is used and can therefore be probed for weakness.



Types of port scans include:

• vanilla - An attempt to connect to all ports (there are 65,536)
• Strobe - An attempt to connect to only selected ports (typically, under 20)
• Stealth scan - Several techniques for scanning that attempt to prevent the request for connection being logged
• FTP Bounce Scan - Attempts that are directed through an File Transfer Protocol server to disguise the cracker's location
• Fragmented Packets - Scans by sending packet fragments that can get through simple packet filters in a firewall
• UDP - Scans for open User Datagram Protocol ports
• Sweep - Scans the same port on a number of computers

cal linux command

Cal command displays a simple calendar.  If arguments are not specified, the current month is displayed.



Syntax :


     cal [options] [[month] year]


cal Options :


     The options of cal command are as follows:


     -1      Display single month output.  (This is the default.)


     -3      Display previous, current and next month output.


     -s      Display Sunday as the first day of the week.  (This is the default.)


     -m      Display Monday as the first day of the week.


     -j      Display Julian dates (days one-based, numbered from January 1).


     -y      Display a calendar for the current year.


Examples :


1) cal 2012
This example gives the calendar of the year 2012.
2) cal -3 9 2011
This example gives an output calendar of one month before and after September 2011.
3) cal 6 2011
It will gives the calender of june 2011.

bc linux command

You may be using bc command in command line for calculations. It can also be used for batch mode calculations as explained below.


Syntax
bc [-c] [-l] [file]




To perform addition using bc command

echo "56.8 + 77.7" | bc

Another way of executing bc calculation

bc <<< 4+2

Let say you want to convert decimal to hexadecimal, you can do this.

echo "obase=16; ibase=10; 56" | bc

A simply way to do calculation with floating point

echo 2/5 | bc -l

Limiting size of emails sent with CPanel?

You can restrict sending and receiving mails which are exceeding a certain size limit with cpanel by adding the line "message_size_limit" on your exim configuration file(exim.conf).



Example:


How can you restrict sending and receiving emails exceeding a size limit of 10MB?

Add this in the first part of your exim.conf file

message_size_limit = 10M

This setting should restrict both incoming and outgoing attachments over 10MB.