gzip, bzip2 are file compression utilities and gunzip, bunzip2 are file uncompression utilities.

bzip2 compresses files to a smaller size compared to gzip, but it takes longer to compress the files. Bzip2 uses Burrows-Wheeler block sorting text compression algorithm (with Huffman coding) and gzip uses Lempel-Ziv coding. so, a file compressed with one utility cannot be uncompressed with the other utility.

Compressing files
$ gzip myfile1.doc.gz
$ bzip2 myfile2.doc.bz2

Now compare the file sizes to verify the reduction in compression ratios.

Uncompressing files
$ gunzip myfile1.doc.gz
$ bunzip2 myfile2.doc.gz

Read more on Gunzip, Unzip, Bunzip with tar

$$ Process ID number of the shell in execution
$? Variable indicating Exit status (from the last command that got executed)
$* Entire argument string in the command line (excluding script name)
$# Number of arguments in the command line (not counting the shell script name)

To access arguments passed to a script, following variables are used.

$0 Name of the program (with entire path)
$1 First argument passed in the command line
$2 Second argument passed in the command line
$n Nth argument passed in the command line

To move or shift to a specified argument in the list of arguments, “shift” command is used. For example, to shift 1 argument just type “shift” or “shift 1” and to shift to the third argument in the list type “shift 2“.

Program: helloargs.sh

#!/bin/bash
echo “Process ID number = $$”
echo “Program name = $0″
echo “Argument string = $*”
echo “Total arguments is $#”
echo “Argument 1 has the value $1″
shift 2
echo “Now argument 1 has the value $1″

Program Results

$ helloargs.sh arg1 arg2 arg3 arg4

Process ID number = 28141
Program name = /tmp/myshells/helloargs.sh
Argument string = arg1 arg2 arg3 arg4
Total arguments is 4
Argument 1 has the value arg1
Now argument 1 has the value arg3

To add a directory to the path, type
$ export PATH=$PATH:/tmp/myshells

The above change is in effect only in the shell from where it is being called

To set the PATH for a specific user, type the below 2 lines in the user’s .bash_profile file ($HOME/.bash_profile).

PATH=$PATH:/tmp/myshells
export PATH

To set the PATH for ALL users except the root user, type the following line in /etc/profile
export PATH=$PATH:/tmp/myshells

Root user is not affected by any setting changes under /etc/profile. To set the path for the root user, we need to modify the root users’ .bash_profile (/root/.bash_profile)

$ sh myshellscript.sh (or)
$ ./myshellscript.sh

If the shell scripts are stored in an executable path, it will save us from typing extra keystrokes. To find the paths that are setup, type

$ echo $PATH
/usr/bin: /usr/local/bin

In order to achieve the desired results, You can
i) save the shell scripts under /usr/local/bin as that path is already listed in the system path
ii) create a folder and add the path to the system path variable

Create a directory “scripts” under /usr/local
$ mkdir/usr/local/scripts

Move the script “myshellscript.sh” to /usr/local/scripts

$ export PATH=$PATH:/usr/local/scripts

Now, type
$ myshellscript.sh
and the shell script will execute as expected.

To set the path permanently, you have to edit .bashrc file.

#!/bin/bash

This is an indication that the shell script is used in bash shell and that it should be called for command execution.

To know the path of the bash shell, type
$ cat /etc/shells

Now, let us begin with “helloshell” shell script

$ vi helloshell

Type in the following lines of the code and save it

# helloshell script
#!/bin/bash
echo “Enter your name …”
read yourname
echo “Welcome $yourname! Path to you SHELL > $SHELL”

Give execute permission to the shell script
$ chmod +x helloshell

To execute the shell script, type
$ . helloshell (or)
$ ./helloshell (or)
$ sh helloshell

TIP 1: To comment any lines in a shell script, use the character ‘#’
TIP 2: To easily identify a shell script, save the file with ‘.sh’

Commonly available shells
- BASH (Bourne Again Shell)
- CSH (C Shell)
- KSH (Korn Shell)

All these shells do the same job and the differences come in the form of syntax that these shells use to execute commands and the built-in functions that comes along with it.

You can find the shell that is in execution by typing
$ echo $SHELL

To find all the available shells in the system, type
$ cat /etc/shells

To add users to the already existing password file, use the command
$ htpasswd -m /secured/.mypass username2

When additional users are added to the password file, ensure that you don’t use the “-c” option.

Read Password protect directories here

The above error message basically means that eth0 i.e your network interface card is not setup correctly.

Some of the commands that can come handy in troubleshooting where the error lies are listed below

$ dmesg | grep eth0
(displays information about the ethernet controller card, mac address etc.)

$ cat /etc/sysconfig/network(displays the network settings)
NETWORKING=yes
HOSTNAME=localhost.localdomain
GATEWAY=192.168.122.1

$ cat /etc/sysconfig/network-scripts/ifcfp-eth0

$ ifconfig eth0 down (to down the nic)
$ ifconfig eth0 up (to start the nic)
$ netstat -rn (display kernel routing IP table)
$ cat ifcfg-eth0 (eth0 configuration settings)
DEVICE=eth0
BOOTPROTO=dhcp
HWADDR=00:16:18:33:B3:CF
ONBOOT=yes

$ service network restart (restart the network settings)

$ ping -c4 google.ca(to check if the internet connection works)

$ traceroute google.ca (to check how the IP navigation path is set)

Refer Red Hat linux 5 configure network article for additional info

$ dpkg-query -s firefox

This is the equivalent of “rpm -qa firefox” in RedHat/CentOS

“Servers should be cautious about depending on URI lengths above 255 bytes because some older client or proxy implementations may not properly support these lengths.”

Different browser agents support different URI length acceptance. In addition, servers too play a role in accepting/denying URI’s over certain length which may either truncate the URI or may give lengthy URI message indications.

It is always advisable to rely on shorter URI’s and to post data when more field sets are to be transferred across web pages.

POST data too has its limit. In this case, it is the size of the data which is controlled by the server settings.

Some interesting discussions:

Limit on query string GET URL parameters