Find IP address on Ubuntu

This post introduces how to find the IP address on your Ubuntu.

Open your terminal (CTRL + ALT + T), and then issue the following command.

$ ifconfig | less

your IP should be listed under

inet addr

Something looks like this:

inet addr: Bcast: Mask:

In this example, would be your IP address.

Note: press “q” to exit.


Some interesting facts about number 0 and 9

Zero is the most complex number among 0-9. It can be nothing, and can also be something that expands other number. For example, append 0 after 1, it expands 1 to another larger number 10.

Number 1-9  just can be something. They don’t expend other number.
Another interesting fact: (I bet most of us know about the first fact, but maybe not the second one…)

  • Any number times 0 goes back to 0 itself.
  • Any number times 9 goes back to 9 itself, no exception.
    2*9 = 18 (1+8 = 9)
    7*9 = 63 (6+3 = 9)

99*9 = 891 (8+9+1 = 18 –> 1+8 = 9)

836*9 = 7524 (7+5+2+4 = 18 –> 1+8 = 9)

2017*9 = 18153 (1+8+1+5+3 = 18 –> 1+8 = 9)

25379*9 = 228411 (2+2+8+4+1+1 = 18 –> 1+8 =9)

After I noticed this interesting fact, I did a search on the Internet.
Why is it that if you multiply any number by nine and add up the digits of the product, they also equal nine? For example 9×12=108. 1+0+8=9

Why is everything multiplied by nine when the digits are added the result is nine? (pdf)

The Magic of Number 9 (pdf)


Shell Scripting Tutorials (Pic-notes)

This post provides some pic notes for Shell Scripting Tutorials (a list of 62 short videos) .

No capital letters in bash shell command (All should be in lowercase)!






touch — create empty (plain text) files

mkdir — create directory (directory is the same thing as folder, in terminal it is called directory, on GUI interface it is called folder)

cd  — change directory


cat > test   — this will create a file called test and write text into the file. (Ctrl+ D to exit editing the file.)

cat < test (shortcut cat test  —  this will open an existing file called test and print its content in the terminal.

cat random test > sample  — this will merge two existing files called random and test, and write the content to a new file called sample

mv — rename (when the source and destination is in the same directory) or move files


rm -r  (r means recursive)

rmdir   — rmdir (only for empty directory)

cp  — copy files and /or directories

ln file1 file 2 — (file 1 is a physical copy of file 2, when file 1 changes, file 2 will be updated automatically and in real time)

ln -s file1 file1_soft  — (file1_soft is a soft link  of file1, when file1 was removed, file1_soft will not work, because the physical copy it refers to has been gone.)




file * command

the out put of wc command:

number of line, word, and charactor in the input file.

wc -l           number of lines

wc -w         number of words

wc -c          number of characters



  • Shell Scripting Tutorial-12: Sort




















-f   file

-d   direcotry

-s file size (greater than zero or not)

-c (character file, e.g.,  .dbf file)

-b (block file like video and image)

-r  (read permission)

-w (write permission)

-x   (executable or not)

cat >> $fname   — will not overwrite the existing content in the file and will append … cat > $fname will overwrite

-n (the length of the string is not zero)

-z (the length of the string is zero)

the output of $?  is 0 means the operation is true, if false, the output is 1.


-a  AND logic operator

because there is a new line symbol, so need to be -eq 2 for a character.




-0  or logic operator

|    pipe symbol

the output of echo $var   as the input of wc -c, use reverse quote symbol to group it as a whole for -eq  operator

|  — the pipe symbol tells the case not to end the case statement after checking [aeiou]*, need to check [AEIOU]* as well for the first case statement.

Normally 0 is for success, 1 is for failure of the result of $?

2 means invalid.

the default IFS is white space.

exec and tty command

exec — exec is a builtin command of the Bash shell. It allows you to execute a command that completely replaces the current process. The current shellprocess is destroyed, and entirely replaced by the command you specify. (more check here.)

tty — Print the file name of the terminal connected to standard input. ( check more at here.)

read — read is a builtin command of the Bash shell, which reads a line of text from standard input and splits it into words. These words can then be used as the input for other commands. (for more info check here.)

sleep command — used for delay (the unit of its parameter is in seconds)

(see also Shell Scripting Tutorial-11: Count Lines, Words & Characters Using ‘wc’)

In this tutorial you’ll learn to iterate over contents of a text file using for loop.


In this tutorial you’ll learn to use nested loops to print all possible combinations of digits 1, 2 and 3.


In this tutorial you’ll learn to use the break statement to transfer control from within the loop to the first statement after the loop.

In this tutorial you’ll learn to use the ‘continue’ statement to skip iterations in loops.


In this tutorial you’ll learn to execute multiple commands on a single line in the terminal without using a shell script.

In this tutorial you’ll learn to add or remove users on the system, also we’ll check out the ‘login’ command to login through a user account in a terminal window.

In this tutorial we’ll build a script that checks for a user every minute and then notifies us when it logs in and also displays the time the user was late in logging in.

In this tutorial we’ll improve the script we built in the previous lesson.

In this tutorial we’ll finish our work on the script we first met in the 56th tutorial. We’ll check every one second for the user and will also report time with greater precision.

In this tutorial you’ll learn to send messages to other users on your system using the ‘write’ command.

You’ll also learn to use ‘finger’ command to see which users who have disabled message reception and which have not.

one terminal (terminal 1):

Another terminal (terminal 2):

in terminal 1:

in Terminal 2:

In Terminal 1:

In terminal 2:

In terminal 1:

In Terminal 2:

EOF — End Of File


Finger command:

In terminal 1:

In Terminal 2:


In Terminal 1:

Note the * before pts/1 (that indicates newuser2 does not allow send msg to it)


In this tutorial you’ll learn to create your own commands using functions.

unset command to remove customized functions in terminal.


In this tutorial we’ll learn to execute a script from another script; not a particularly hard thing to do but I recorded a tutorial on it anyway.

Thank you so much for watching the tutorials in this course. I hope you guys had as much fun watching the tutorials as I had recording them for you.

The END:) happy shell scripting!








Use latexdiff to compare different versions of LaTeX tex files to visually mark differences

This post introduces how to use latexdiff to compare two versions of latex *.tex files, in order to visually mark changes to tex files.

(One colleague asked me about the usage of latexdiff, so I think it is a good time to write it up into a blog post to share with those who need some help with this.)

latexdiff is a Perl script and requires an installation of Perl 5.8 or higher.

Use latexdiff to mark differences and create a new LaTeX document with markup differences between two latex files.

  • Linux (Ubuntu) users

latexdiff can be found in Ubuntu repository and detailed description of usage can be found here.

In your terminal, issue the commands below to install latexdiff.

sudo apt-get update
sudo apt-get install latexdiff
# open your terminal, and cd to the tex files you need to compare
# issue the following command
latexdiff original_version.tex revised_version.tex > diff.tex

Run the diff.tex file like you run a normal tex file, you will see beautifully marked differences in the pdf file.

  • Mac OS X users
latexdiff is a Perl script and requires an installation of Perl 5.8 or higher.
Mac OS X includes an installation of Perl and no additional setup is required. latexdiff can be found in any CTAN repository
(If you are using MacTex on your mac,  latexdiff is already installed in MacTex under /usr/texbin/latexdiff. No need to install it separately. To use latexdiff in commend line, see below)
After you setting perl and latexdiff, just open your terminal, and cd to the path where you put you two versions of latex tex files, and then issue the command. (If the two tex files are located in different location, you can just give the relative path to that location for each version of tex file.)


$ latexdiff original_version.tex revised_version.tex > diff.tex
will compare original_version.tex to revised_version.tex and create a new file diff.tex showing the differences.
Run the diff.tex file like you run a normal tex file, you will see beautifully marked differences in the pdf file.
  • Online latexdiff 
when you get the diff.tex on the online latexdiff, you can paste into a new tex file and then you can run it and you will see the pdf which shows the differences.
  • Multiple tex files for each version

If each version of your text files contains multiple (sub) tex files or if you want to use latexdiff with Git, check the reference lists for potential solutions.


LATEXDIFF: SUPERB DIFF TOOL FOR LATEX (pdf) — talks about solutions to multiple files, but this can only deal with sub-files on the same level, not nested cases.

Two LaTeX gems: ShareLaTeX and latexdiff (pdf)

Using Latexdiff For Marking Changes To Tex Documents (pdf)

Multiple-file LaTeX diff (pdf— this one is very good. – use python code to merge multiple tex files to one big tex file for each version and then use latexdiff to make comparision.

Latexdiff with subfiles (pdf) — this one talks about solutions to multiple tex files.

Using latexdiff with git (pdf) – Git Latexdiff  — this is about using git together with latexdiff

Git and latexdiff: compare versions of LaTeX documents (pdf) — this is also about using git together with latexdiff




Bash shell scripting resources

This post provides some useful (video) resources for bash shell scripting. (If you do not like video style tutorials, check here for a post that is not video based.)

The bash ( Bourne Again Shell) is the most common shell installed with Linux distributions and Mac OS.

  • Bash Basics (A video playlist with 9 videos) — The Basics of using Bash in Linux with a focus on Ubuntu.

Bash Basics Part 1 of 8 | Access and Navigation

       $ means you logged in as a normal user.

       # means logged in as a root user.

Bash Basics Part 3 of 8 | Privileges and Permissions

Bash Basics Part 8 of 8 | Bash Scripting

[LaTeX] Comment out some text in a bib file

This post introduces how to comment out some text in a .bib file.

Sometimes we need to keep some information associated with a paper, such as web link where we can find the file, but we do not want it appears in the references.

It said that “//” or “%” works for some cases, but apparently it does not work for me. My solution is use comment = {add the info you wanted to comment with this  pair of curly brackets},

See below for an example.

title={paper title here},
author={author name},