PS1 : Customize bash custom prompt
So how do you setup, change and pimp out Linux / UNIX shell prompt?
Most of us work with a shell prompt. By default most Linux distro displays hostname and current working directory. You can easily customize your prompt to display information important to you. You change look and feel by adding colors. In this small howto I will explain howto setup:
a] Howto customizing a bash shell to get a good looking prompt
b] Configure the appearance of the terminal.
c] Apply themes using bashish
d] Howto pimp out your shell prompt
Prompt is control via a special shell variable. You need to set PS1, PS2, PS3 and PS4 variable. If set, the value is executed as a command prior to issuing each primary prompt.
* PS1 - The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt string. The default value is \s-\v\$ .
* PS2 - The value of this parameter is expanded as with PS1 and used as the secondary prompt string. The default is >
* PS3 - The value of this parameter is used as the prompt for the select command
* PS4 - The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays during an execution trace. The first character of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is +
How do I display current prompt setting?
Simply use echo command, enter:
$ echo $PS1
Output:
\\u@\h \\W]\\$
How do I modify or change the prompt?
Modifying the prompt is easy task. Just assign a new value to PS1 and hit enter key:
My old prompt --> [pratyush@pratyush-laptop ~]$
PS1="touch me : "
Output: My new prompt
touch me :
So when executing interactively, bash displays the primary prompt PS1 when it is ready to read a command, and the secondary prompt PS2 when it needs more input to complete a command. Bash allows these prompt strings to be customized by inserting a number of backslash-escaped special characters that are decoded as follows:
* \a : an ASCII bell character (07)
* \d : the date in "Weekday Month Date" format (e.g., "Tue May 26")
* \D{format} : the format is passed to strftime(3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation. The braces are required
* \e : an ASCII escape character (033)
* \h : the hostname up to the first '.'
* \H : the hostname
* \j : the number of jobs currently managed by the shell
* \l : the basename of the shell’s terminal device name
* \n : newline
* \r : carriage return
* \s : the name of the shell, the basename of $0 (the portion following the final slash)
* \t : the current time in 24-hour HH:MM:SS format
* \T : the current time in 12-hour HH:MM:SS format
* \@ : the current time in 12-hour am/pm format
* \A : the current time in 24-hour HH:MM format
* \u : the username of the current user
* \v : the version of bash (e.g., 2.00)
* \V : the release of bash, version + patch level (e.g., 2.00.0)
* \w : the current working directory, with $HOME abbreviated with a tilde
* \W : the basename of the current working directory, with $HOME abbreviated with a tilde
* \! : the history number of this command
* \# : the command number of this command
* \$ : if the effective UID is 0, a #, otherwise a $
* \nnn : the character corresponding to the octal number nnn
* \\ : a backslash
* \[ : begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt
* \] : end a sequence of non-printing characters
Let us try to set the prompt so that it can display today’d date and hostname:
PS1="\d \h $ "
Output:
Sat Jun 02 server $
Now setup prompt to display date/time, hostname and current directory:
$ PS1="[\d \t \u@\h:\w ] $ "
Output:
[Sat Jun 02 14:24:12 pratyush@pratyush-laptop:~ ] $
How do I add colors to my prompt?
Let us say when you login as root/superuser, you want to get visual confirmation using red color prompt. To distinguish between superuser and normal user you use last character in the prompt, if it changes from $ to #, you have superuser privileges. So let us set your prompt color to RED when you login as root, otherwise display normal prompt.
Open /etc/bashrc (Redhat and friends) / or /etc/bash.bashrc (Debian/Ubuntu) or /etc/bash.bashrc.local (Suse and others) file and append following code:
# vi /etc/bashrc
or
$ sudo gedit /etc/bashrc
Append the code as follows
# If id command returns zero, you’ve root access.
if [ $(id -u) -eq 0 ];
then # you are root, set red colour prompt
PS1="\\[$(tput setaf 1)\\]\\u@\\h:\\w #\\[$(tput sgr0)\\]"
else # normal
PS1="[\\u@\\h:\\w] $"
fi
Close and save the file.
Tuesday, November 23, 2010
Monday, November 15, 2010
What are Wild Pointers? How can we avoid?
Uninitialized pointers are known as wild pointers because they point to some arbitrary memory location and may cause a program to crash or behave badly.
view source
print?
int main()
{
int *p; /* wild pointer */
*p = 12; /* Some unknown memory location is being corrupted. This should never be done. */
}
Please note that if a pointer p points to a known variable then it’s not a wild pointer. In the below program, p is a wild pointer till this points to a.
view source
print?
int main()
{
int *p; /* wild pointer */
int a = 10;
p = &a; /* p is not a wild pointer now*/
*p = 12; /* This is fine. Value of a is changed */
}
If we want pointer to a value (or set of values) without having a variable for the value, we should explicitly allocate memory and put the value in allocated memory.
view source
print?
int main()
{
int *p = malloc(sizeof(int));
*p = 12; /* This is fine (assuming malloc doesn't return NULL) */
}
view source
print?
int main()
{
int *p; /* wild pointer */
*p = 12; /* Some unknown memory location is being corrupted. This should never be done. */
}
Please note that if a pointer p points to a known variable then it’s not a wild pointer. In the below program, p is a wild pointer till this points to a.
view source
print?
int main()
{
int *p; /* wild pointer */
int a = 10;
p = &a; /* p is not a wild pointer now*/
*p = 12; /* This is fine. Value of a is changed */
}
If we want pointer to a value (or set of values) without having a variable for the value, we should explicitly allocate memory and put the value in allocated memory.
view source
print?
int main()
{
int *p = malloc(sizeof(int));
*p = 12; /* This is fine (assuming malloc doesn't return NULL) */
}
Sunday, November 14, 2010
Little and big endian
What are these?
Little and big endian are two ways of storing multibyte data-types ( int, float, etc). In little endian machines, last byte of binary representation of the multibyte data-type is stored first. On the other hand, in big endian machines, first byte of binary representation of the multibyte data-type is stored last.
Suppose integer is stored as 4 bytes (For those who are using DOS based compilers such as C++ 3.0 , integer is 2 bytes) then a variable x with value 0×01234567 will be stored as following.
Memory representation of integer ox01234567 inside Big and little endian machines
How to see memory representation of multibyte data types on your machine?
Here is a sample C code that shows the byte representation of int, float and pointer.
view source
print?
#include
/* function to show bytes in memory, from location start to start+n*/
void show_mem_rep(char *start, int n)
{
int i;
for (i = 0; i < n; i++)
printf(" %.2x", start[i]);
printf("\n");
}
/*Main function to call above function for 0x01234567*/
int main()
{
int i = 0x01234567;
show_mem_rep((char *)&i, sizeof(i));
getchar();
return 0;
}
When above program is run on little endian machine, gives “67 45 23 01″ as output , while if it is run on endian machine, gives “01 23 45 67″ as output.
Is there a quick way to determine endianness of your machine?
There are n no. of ways for determining endianness of your machine. Here is one quick way of doing the same.
view source
print?
#include
int main()
{
unsigned int i = 1;
char *c = (char*)&i;
if (*c)
printf("Little endian");
else
printf("Big endian");
getchar();
return 0;
}
In the above program, a character pointer c is pointing to an integer i. Since size of character is 1 byte when the character pointer is de-referenced it will contain only first byte of integer. If machine is little endian then *c will be 1 (because last byte is stored first) and if machine is big endian then *c will be 0.
Does endianness matter for programmers?
Most of the times compiler takes care of endianness, however, endianness becomes an issue in following cases.
It matters in network programming: Suppose you write integers to file on a little endian machine and you transfer this file to a big endian machine. Unless there is little andian to big endian transformation, big endian machine will read the file in reverse order. You can find such a practical example here.
Standard byte order for networks is big endian, also known as network byte order. Before transferring data on network, data is first converted to network byte order (big endian).
Sometimes it matters when you are using type casting, below program is an example.
view source
print?
#include
int main()
{
unsigned char arr[2] = {0x01, 0x00};
unsigned short int x = *(unsigned short int *) arr;
printf("%d", x);
getchar();
return 0;
}
In the above program, a char array is typecasted to an unsigned short integer type. When I run above program on little endian machine, I get 1 as output, while if I run it on a big endian machine I get 256. To make programs endianness independent, above programming style should be avoided.
What are bi-endians?
Bi-endian processors can run in both modes little and big endian.
What are the examples of little, big endian and bi-endian machines ?
Intel based processors are little endians. ARM processors were little endians. Current generation ARM processors are bi-endian.
Motorola 68K processors are big endians. PowerPC (by Motorola) and SPARK (by Sun) processors were big endian. Current version of these processors are bi-endians.
Does endianness effects file formats?
File formats which have 1 byte as a basic unit are independent of endianness e..g., ASCII files . Other file formats use some fixed endianness forrmat e.g, JPEG files are stored in big endian format.
Which one is better — little endian or big endian
The term little and big endian came from Gulliver’s Travels by Jonathan Swift. Two groups could not agree by which end a egg should be opened -a-the little or the big. Just like the egg issue, there is no technological reason to choose one byte ordering convention over the other, hence the arguments degenerate into bickering about sociopolitical issues. As long as one of the conventions is selected and adhered to consistently, the choice is arbitrary.
Little and big endian are two ways of storing multibyte data-types ( int, float, etc). In little endian machines, last byte of binary representation of the multibyte data-type is stored first. On the other hand, in big endian machines, first byte of binary representation of the multibyte data-type is stored last.
Suppose integer is stored as 4 bytes (For those who are using DOS based compilers such as C++ 3.0 , integer is 2 bytes) then a variable x with value 0×01234567 will be stored as following.
Memory representation of integer ox01234567 inside Big and little endian machines
How to see memory representation of multibyte data types on your machine?
Here is a sample C code that shows the byte representation of int, float and pointer.
view source
print?
#include
/* function to show bytes in memory, from location start to start+n*/
void show_mem_rep(char *start, int n)
{
int i;
for (i = 0; i < n; i++)
printf(" %.2x", start[i]);
printf("\n");
}
/*Main function to call above function for 0x01234567*/
int main()
{
int i = 0x01234567;
show_mem_rep((char *)&i, sizeof(i));
getchar();
return 0;
}
When above program is run on little endian machine, gives “67 45 23 01″ as output , while if it is run on endian machine, gives “01 23 45 67″ as output.
Is there a quick way to determine endianness of your machine?
There are n no. of ways for determining endianness of your machine. Here is one quick way of doing the same.
view source
print?
#include
int main()
{
unsigned int i = 1;
char *c = (char*)&i;
if (*c)
printf("Little endian");
else
printf("Big endian");
getchar();
return 0;
}
In the above program, a character pointer c is pointing to an integer i. Since size of character is 1 byte when the character pointer is de-referenced it will contain only first byte of integer. If machine is little endian then *c will be 1 (because last byte is stored first) and if machine is big endian then *c will be 0.
Does endianness matter for programmers?
Most of the times compiler takes care of endianness, however, endianness becomes an issue in following cases.
It matters in network programming: Suppose you write integers to file on a little endian machine and you transfer this file to a big endian machine. Unless there is little andian to big endian transformation, big endian machine will read the file in reverse order. You can find such a practical example here.
Standard byte order for networks is big endian, also known as network byte order. Before transferring data on network, data is first converted to network byte order (big endian).
Sometimes it matters when you are using type casting, below program is an example.
view source
print?
#include
int main()
{
unsigned char arr[2] = {0x01, 0x00};
unsigned short int x = *(unsigned short int *) arr;
printf("%d", x);
getchar();
return 0;
}
In the above program, a char array is typecasted to an unsigned short integer type. When I run above program on little endian machine, I get 1 as output, while if I run it on a big endian machine I get 256. To make programs endianness independent, above programming style should be avoided.
What are bi-endians?
Bi-endian processors can run in both modes little and big endian.
What are the examples of little, big endian and bi-endian machines ?
Intel based processors are little endians. ARM processors were little endians. Current generation ARM processors are bi-endian.
Motorola 68K processors are big endians. PowerPC (by Motorola) and SPARK (by Sun) processors were big endian. Current version of these processors are bi-endians.
Does endianness effects file formats?
File formats which have 1 byte as a basic unit are independent of endianness e..g., ASCII files . Other file formats use some fixed endianness forrmat e.g, JPEG files are stored in big endian format.
Which one is better — little endian or big endian
The term little and big endian came from Gulliver’s Travels by Jonathan Swift. Two groups could not agree by which end a egg should be opened -a-the little or the big. Just like the egg issue, there is no technological reason to choose one byte ordering convention over the other, hence the arguments degenerate into bickering about sociopolitical issues. As long as one of the conventions is selected and adhered to consistently, the choice is arbitrary.
Wednesday, November 10, 2010
TALKING LINUX MACHINE... (tested in Ubuntu)
Hello,
You can play with your linux machine. It can talk for you what you want. May be you know the Linux utility espeak. It can be used to listen the voice as we want. if you type espeak "hi buddy " , in the terminal your computer will speak it.
Another funny thing you can do it with it...! if you append the following code to the espeak "Thanks for login" to .bash_login inside your home directory( /home/your_name/.bash_logout). Then whenever you login via terminal(not GUI), it will speak like that.
Also you can add espeak "Open terminal" or any message you want it to /home/your_name/.bashrc file. Next see the result while opening new terminal.
However there various place that you can add it and make your Linux machine talk............!
You can play with your linux machine. It can talk for you what you want. May be you know the Linux utility espeak. It can be used to listen the voice as we want. if you type espeak "hi buddy " , in the terminal your computer will speak it.
Another funny thing you can do it with it...! if you append the following code to the espeak "Thanks for login" to .bash_login inside your home directory( /home/your_name/.bash_logout). Then whenever you login via terminal(not GUI), it will speak like that.
Also you can add espeak "Open terminal" or any message you want it to /home/your_name/.bashrc file. Next see the result while opening new terminal.
However there various place that you can add it and make your Linux machine talk............!
Friday, November 5, 2010
First line of my Blog
1. If u born poor it is not ur mistake, BUT if u die poor it is ur mistake ......................................................................... Bill gates
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