Cut and paste text files by columns

How many times have you wanted to edit those results files columnwise?

Here’s a generic UNIX way: (startcol and endcol refer to character columns, including whitespace. Tab counts as 1.)

cut -cstartcol-endcol file {>receiving file} paste sourcefile1 sourcefile2 {>final file}

I like to preview and fine-tune my commands by looking at the result on standard out (the shell window) and then redirecting output to a (temporary) file when things look right. Those feeling really adventurous can combine things into a single command line…

Great Unix Tips & Tricks: Vol 1

Whether you are a newbie to Linux or you are a seasoned guru, you are bound to find some scripts or programs which just make your life on the computer exponentially easier. The following is a list of them and explains what they do.

1. tac

tac is a command similar to cat, except that it reverses the output of the specified file.

tac

2. Multiple X-Sessions

Using multiple X-Sessions allows a user to have open more than just one instance of XFree, so multiple desktop environments can be used at a time.

startx -- :

terminal name is simply which terminal X should start on. By default it starts on

0:0. If you want two open, you may want it open on 0:1, which can be run by:

startx -- :1

The resulting desktops will exist on F7 - F12.

3. links

links is a text based browser which has full support for tables

links

is simply the hostname or IP address of the computer to connect to.

sparse files

Due to the Unix concept of sparse files, you can create seemingly enormous files that in reality take next to no disk space. The following program will create a 305,419,897 byte file called 'core' that may result in you receiving a 'cleanup' email message from a less-than-seasoned Unix sysadmin even though it occupies virtually no real disk space.

    $ cat bigcore.c
    #include 
    #include 

    int main(void) {
        int fd = open("core", O_CREAT|O_WRONLY|O_TRUNC, 0600);
        lseek(fd, 0x12345678, SEEK_SET);
        write(fd, "1", 1);
        close(fd);
        return 0;
    }

    $ cc -o bigcore bigcore.c
    $ ./bigcore
    $ ls -l core
    -rw-------  1 dmr  staff  305419897 May  1 03:50 core
    $ du -k core
    48      core
    $

Ctrl+D or exit: Which one is better?

In BASH, to exit from shell we can use Ctrl+D or exit ? which one is better?

-Ctrl+D conveniently exits Bash
-Sometimes too conveniently bcoz session may be terminated if pressed accidentaly.

To avoid the accidental exit, we can specify the no of "Ctrl+D" pressed before exit.

$export IGNOREEOF=1

$ Ctrl+D
$ Use "exit" to leave the shell.
$ Ctrl+D
$ exit

Games

If you like to use Linux and only Linux, and you also like to play games on your computer, the best choice is to buy native Linux games (games that run without any intermediate layer on your Linux system). You can find lots of information about available games and even order them through the links below.

Naturally there are also lots of free games, many simple but fun games are included with Linux distributions, but some are not, so I have included links about those too.

Linux Game Publishing
Tux Games
Linux Gaming
Linuxgames
The Linux Game
Mandrake eXPerience FREE GAMES section
Article on Gaming and Linux in 2003

Command based Calculator in BASH

Bash can perform simple arithmetic operations at the command prompt.

To use this feature, simply type in the arithmetic expression you wish to evaluate at the prompt within double parentheses, as illustrated below. Bash will attempt to perform the calculation and return the answer.


bash> echo $((16/2))
8

bash> echo $((16*2))
32

Open source I use and recommend

I'm always happy to put in a plug for the tools I use every day. With the exception of the Anti-Spyware category, all of the following software is not only free, but open source as well. You might have heard of, or even use, most of these programs already, but give the list a scan for any hidden gems you might not have encountered before.

Virtual Desktop
Organize your applications over several virtual Windows desktops. In this "once you've tried it, you can't live without it" category, we have two strong contenders: VirtuaWin and Virtual Dimension. The latter application has some extra features, but it's a virtual (sorry) tie: you can't lose trying either one of them. [Windows Only]

File Archiver
7-Zip can create ZIP, GZIP, BZIP2 and TAR archives, and can unpack RAR, CAB, even your old LZH and ARJ files. And the Windows version integrates into Windows Explorer. What's not to like?

Server Operating System
I am always happy to put in a word for my favourite Unix server operating system, FreeBSD. While GNU/Linux may have better support for bleeding-edge hardware, I find FreeBSD more stable, scalable, secure and usable. When you consider all the various Linux distributions out there, the fact that there is one FreeBSD is, frankly, less confusing. The excellent ports system and the fact that FreeBSD can run Linux binaries are icing on the cake.

X Server
If you need to run a particular X application on Windows, or if you prefer xterm to putty, you need Cygwin/X. It's a large download, and there are decent commercial alternatives in this category, but this is my pick. [Windows Only]

Web Browser and Email
Yes, Firefox is slowly but surely winning the browser war, so I don't need to plug it a whole lot. Thunderbird will probably do the same thing to Outlook eventually, once more people try this full-featured (spell-checking, address book, spam filter, etc.) email client that provides the same slick email interface whether you're running Linux, Windows or OS X. Highly recommended.

Database
Which one is best: MySQL or PostgreSQL? The debate rages on, but the two have recently come much closer in terms of capabilities, with MySQL playing catch-up in terms of features (stored procedures, triggers, views, etc.). It's getting tougher and tougher to pick a winner, but I think they each have their strengths and appropriate place, and quite honestly, I'm as happy as a clam using either one.

SFTP Client
If you need to securely transfer files between a Windows client and a remote computer running SSH, and you want a GUI interface, FileZilla will make your life easier. [Windows Only]

Text Editor
If you know what a text editor is, you probably already have a favourite, and if past experience is any guide, you probably will use it exclusively the rest of your days. The one they will pry from my cold, dead fingers is Vim.

Terminal Emulator
If you work on Windows, but need to access remote Unix accounts, how can you not already love PuTTY? If only they would finish the Mac OS X port, I could die happy. Until that day, I'll use iTerm on OS X, if only for the tabbed windows. In conjunction with either of these terminal emulators, I could not work without GNU Screen, a console window manager that allows you to run interactive shells in multiple virtual terminals.

Argument of the previous command

To use the last argument of the previous command line as an argument, use $_.

For example:
$ ls -l /home/sanju/somefile
$ vi $_

It will open /usr/home/somefile using vi editor.

Core dump information

To determine what program dumped an anonymous core file, use gdb:
----------
$ gdb -core core
Core was generated by `a.out'.
Program terminated with signal 6, Abort trap.
----------

If you are on a system without gdb, try the file command:
----------
$ file core
core: ELF 32-bit LSB core file of 'a.out' (signal 6)
$
----------

Limiting File Size in UNIX

C shell
limit filesize 2m

Korn/Bash shell

ulimit -f max-size
ex: Korn/Bash shell limit 2 MB.
ulimit -f 2000

Removing non-consecutive duplicate lines from a file

The uniq command will "Discard all but one of successive identical lines" from a file or input stream.

In order to remove non-consecutive duplicate lines, use awk:

awk '!x[$0]++' FILE

A list of directories

To see a list of subdirectories of the current directory:

printf "%s\n" */

With a Bourne shell:

echo */.

The Easy PATH

To format one's PATH variable for easy viewing, try this function:
==============

path()
{
oldIFS=$IFS
IFS=:
printf "%s\n" $PATH
IFS=$oldIFS
}
==============

A typical run of the function:
$ path
/bin
/usr/bin
/usr/bin/X11
/usr/X11R6/bin
/usr/local/bin
/home/sanju/bin
/home/sanju/scripts

Linux: How to Encrypt and decrypt files with a password

To encrypt and decrypt files with a password, use gpg command. It is an encryption and signing tool for Linux/UNIX like operating system such as FreeBSD/Solaris.

GnuPG stands for GNU Privacy Guard and is GNU's tool for secure communication and data storage. It can be used to encrypt data and to create digital signatures. It includes an advanced key management facility.

Encrypting a file in linux
To encrypt single file, use command gpg as follows:

$ gpg -c filename

To encrypt myfinancial.info file, type the command:$ gpg -c myfinancial.info

Output:

Enter passphrase:
Repeat passphrase:

This will create a myfinancial.info.gpg file.

Option:
-c : Encrypt with symmetric cipher

Caution if you ever forgot your password aka passphrase, you cannot recover the data as it use very strong encryption.