Usual tiny skills & solutions

Ubuntu and Win10 - double OS

2016-02-21

Yesterday I helped my friend install Ubuntu (14.04 LTS) on his PC where there has been a MS Win10.
I used UltraISO to create a U-disk installation and installed it.
However, then we could only start up Ubuntu and there was no Win10.
Then we use LaoMaoTao software to rescue Win10's loader, while the Ubuntu disappeared.
Then we use EasyBCD on Win10 to add an Ubuntu loader.
Bingo!

Modify a binary file in Linux

2016-02-28

Here is a helloworld.c:

#include <stdio.h>

int main()
{
    printf("Hello, world\n");
    return 0;
}

gcc -o helloworld helloworld.c

Then I want to change the string "Hello, world" to "hello, world".

Here is :

vim helloworld -b      # -b means binary
:%!xxd                 # observe in hex mode
# move cursor to letter 'H' and change it to 'h'
:%!xxd -r              # recover from text to binary
:wq

bingo!

Get file-size on Linux

2016-02-29

Here is an interesting question that how can I get the file size in Linux environment.

Method.1

#include <stdio.h>

FILE *fp = fopen(filename, "r");
if(!fp)
    return -1;
fseek(fp, 0L, SEEK_END);
int size=ftell(fp);
printf("size is %d\n", size);
fclose(fp);

However, this method must load the whole file into memory. If the file is very large, its speed will be ridiculous!

Method.2

#include <sys/stat.h>
#include <stdio.h>

struct stat statbuf;
int state = stat(filename, &statbuf);
if(state < 0)
    return -1;
int size = statbuf.st_size;
printf("size is %d", size);

Bingo!

Win7 installation - GPT & MBR - Problems

2016-02.29

Today my friend want to install Win7. I use a U-disk with LaoMaotao software and Win7 GHO. However, his disk is GPT mode.
The Win7 should be installed in MBR mode by default.
So we use LaoMaotao to change the disk into MBR mode.

P.S. Maybe we should also study how to install Win7 in GPT mode for the reason that GPT will be in fashion from now on.

bingo!

Editors in Linux automatically add '\n' to a file

2016-03-01

Today I use Vim to create a text file and store it. Then I use 'xxd' to observe that file. Here is a '\x0a' at the end of the text.
And gedit will do so. Somebody says that is because of the canonical mode in Linux.

<Backspace> makes no effect in Vim & Vi

2016-03-01

Today my friend uses Vi to edit a text. However he can't use to delete letters.
Here is the solution(Copy from Internet):

set nocompatible
set backspace=indent,eol,start

UTF-8 & Unicode & ASCII

2016-03-02

execve - C function

2016-03-03

Today I read CSAPP Chapter-8 and follow it to make a simple shell.
Here's the code:

        if(pid == 0){ // child runs user job
            if(execve(argv[0], argv, environ) < 0){
                printf("evecve error: %s\n", strerror(errno));
                printf("%s: Command not found.\n", argv[0]);
            }
        }

However, I can't use [echo "hello,world"] to execute [echo] instruction.
I must use [/bin/echo "hello,world"]. I have deliver the environment variables into the sub-process.
So, how can that thing happen?
After searching on the Internet, I know it.
[execve] belongs to a [exec] family. And there are some suffixes:

l // means that you must deliver the argv[0], argv[1]... one by one as variables and arguments end up with a NULL.
v // means that you should give a two-dimension pointer [**argv] as a variable. So you can't use suffix [v] and [l] at the same time.
p // means that you can use the $PATH environment variable to search for the argv[0] (filename).
e // means that you can deliver a [**envp] to give an environment to sub-process. Otherwise, the sub-process will use parent-process' environment.

Here are some functions:

execl
execv
execle
execlp
execvp
execve
execvpe

For the small shell above, we should use [execvpe] to replace [execve].
P.S.

for more details, please refer to [man execl] in Linux.

bingo!

stdin/stdout/stderr & STDIN_FILENO/STDOUT_FILENO/STDERR_FILENO

2016-03-04

When I read CSAPP Chapter-8 there is a statement:

if((n = read(STDIN_FILENO, buf, sizeof(buf))) < 0)

What is [STDIN_FILENO] ?

[man read]
ssize_t read(int fd, void *buf, size_t count);

So STDIN_FILENO/STDOUT_FILENO/STDERR_FILENO are file descriptors.
They are [int] type.
And they are from <unistd.h>

Then, what about stdin/stdout/stderr ?

[man fread]
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

So stdin/stdout/stderr are pointers.
They are [FILE *] type.
And they are from <stdio.h>

P.S.
[A] stdin/stdout/stderr
[B] STDIN_FILENO/STDOUT_FILENO/STDERR_FILENO

[A] belong to C standard library.
[B] belong to OS API library.
C standard library packages the OS API.
[A] are related to functions like fread,fwrite,fclose...
[B] are related to functions like read,write,close...(system call)

Bingo!

Go Into Console Interface Directly After startup [Linux]

2016-03-04

This is a skill from my notebook.
environment: Ubuntu 14.04 LTS

sudo gedit /etc/default/grub

Find GRUB_CMDLINE_LINUX_DEFAULT
The key may be

"quiet splash"

Change it to

"quiet splash text"

Then

sudo update-grub

How to go into GUI when you are in text console:

sudo start lightdm

How to switch off GUI interface:

sudo stop lightdm

sizeof('A') Amazing !

2016-03-04

This is a discovery from my notebook.
environment: Ubuntu 14.04 LTS

sizeof('A')
char a = 'A'
sizeof(a)
gcc...

Result will be 4, 1

g++...

Result will be 1, 1

Reason: The C language regards character constant as [int] type.

What is *nix ?

2016-03-05

I have seen *nix for many times. I thought it was Unix and for some odd reasons it was writen as *nix.

But now I know that the * is a wildcard.
And the *nix means Unix and OS like Linux......

bingo!

Learn a new language as a professor

2016-03-05

Today I begin to learn to use Python as a professor.
That means I plan to learn a new language without zero-based experience.
I have learnt C. And you know that for most languages there are just these things:

Data type or structure, control-instruction, other syntax.

Just read a manual, and practice, make mistakes, practice and make mistakes.
Then you will be familiar with it.
Don't simply learn it. Use it. Love it.

Why Can't I Use mov [mem], [mem] ?

2016-03-09

I have learnt Assembly language for some time.
It confuses me for a long time that why the instruction mov [mem], [mem] is invalid.
Somebody says that it is because the designers of Intel CPU didn't design such a instruction.
But Why they didn't?
I believe that they must know that idea at least.
And they designed what we use today for some reasons I think.
Why?

No bool Type in C ?

2016-03-09

Today when I use gcc to compile a .c file it informed me that there was no bool* type.....
Oh, my god. How can that be!
Then I search for the answer on the Internet and look up in K&R.
It seems that there is no bool type in C.......It is in C++......
So?

You can use some methods to make it:

// method 1
#include <stdbool.h>  // from C99

Or, just use g++ (method 2)

Also, you can use

// method 3
typedef int BOOL;
#define TRUE 1
#define FALSE 0
// method 4
typedef enum{TRUE = 1, FALSE = 0}BOOL;

bingo!

Copy/Past Shortcuts in Linux console

2016-03-11

Ctrl + Shift + C // Copy
Ctrl + Shift + V // Paste

Use Wget and Bash to Backup My Blog

2016-03-12

I want to backup blogs in cnblogs.
With the idea and help of my friend, here is the method:

First, you need to learn some wget-options:

-r # recursive
-b # background
-o FILE # log all messages to FILE (to std error by default)
-a FILE # similart to -o, while it is append
--quiet # turn off output
-i # you can select a text of URLs to download
--spider # Do not download; Just check
-x # --force-directories
-l depth # recursive depth
-np # no parent websites (when recursive)
-k # change the links for local viewing
-P PATH # specify the download directory
--accept-regex urlregex # use regular expressions to indicate what websites you want to be downloaded
--reject-regex urlregex # object functions to option --accept-regex urlregex
-p # --page-requisites ; download all the elements to show a website

Now, we use a bash script:

#!/bin/bash

# get date and time
x=$(date +back%Y%m%d%H%M) 

# make a directory named with data and time
mkdir $x  

# back up blog to the directory above
wget -r -p -k -x --accept-regex "http://www.cnblogs.com/00100011F" \
    http://www.cnblogs.com/00100011F -P $x

# download images in articles
d1='w*'
d2='00100011F'
d3='p'
d4='blogimages'
cd $x
cd $d1
cd $d2
cd $d3
mkdir $d4
cat * | grep img | grep -v 'logo' | cut -d '"' -f 2 | xargs wget -P $d4

P.S.

here is another wonderful instruction:
wget -r -p -np -k -P Path URL # to download a whole website

bingo!

Configure Brightness on Ubuntu

2016-03-31

The brightness on Ubuntu is 100 by default.
You can configure it and don't bother to change it by yourself each time you turn on your laptop.

Note that

/sys/class/backlight/acpi_video0/brightness

will dynamically record current brightness.

And

/etc/rc.local

This script is executed at the end of each multiuser runlevel.

You can add one instruction here before exit 0:

sudo echo 52 > /sys/class/backlight/acpi_video0/brightness
exit 0 # (This instruction is original there)

bingo!

Use GPG to Encrypt/Sign

2016-04-21

Layer Mind

2016-04-29

Markdown2PDF

2016-07-12

Long time no see.
Today I want to transform markdown file into PDF format. So I use the print function of firefox. First you can export the markdown file to HTML format. Then open HTML file in firefox and choose file -> print -> print to file. (In fact my markdown editor haroopad has print function like firefox itself so I can transform the file directly...)
: )