Linux World: December 2013

2014 on linux terminal

The following script,when executed, will create 2014 on the linux terminal.

#!/bin/bash 

str_dwn() {
rows=$1
s_start=$2
char=$4
str_st_row=$3
rows_end=$((str_st_row+rows))
for((i=str_st_row;i<rows_end;i++))
do 
tput cup $i $s_start
printf "$char " 
done
echo""
}

backslash() {
b_st_row=$1
b_st_cols=$2
b_char=$3
b_r_end=$((b_st_row + 5 ))
for((p=b_st_row;p<b_r_end;p++))
do 
tput setaf $4
tput cup $p $b_st_cols
printf "$b_char " 
((b_st_cols++))
done
echo""
}

forward() {
f_st_row=$1
f_st_cols=$2
f_char=$3
f_r_end=$((f_st_row + 5))

for((i=f_st_row;i<f_r_end;i++))
do 
tput cup $i $f_st_cols
printf "$f_char "
 ((f_st_cols--))
done
echo""

}


horizontal() {
hrow=$2
hstart=$1
hcols=$3
char=$4

end=$((hstart + hcols))

for((i=hstart;i<=end;i++))
do 
tput cup $hrow $i
printf "$char" 
done
echo""
}

gen_rand() {
num1=$1

if [ $num1 -eq 0 ]
then
temp=$(($RANDOM % 2))
rand=$((temp + 2))

else

temp=$(($RANDOM % 5))
rand=$((temp + num1))
fi
}


two() {

hstart_2=$1
hrow_be=$2
hcols_2=$3
char=$4
color=$5

set_color

horizontal $hstart_2 $hrow_be $hcols_2 "$char"

st_row_2=$((hrow_be+1))
f_st_cols_2=$((hstart_2+hcols_2))

forward $st_row_2 $f_st_cols_2 "$char" $color
hstart_2_lo=$start
hrow_2_lo=$((hrow_be+6))
hcols_2_lo=5
horizontal $hstart_2_lo $hrow_2_lo $hcols_2_lo "$char" 

}



zero() {

set_color

hstart_0=$((f_st_cols_2 +4))
hcols_0=5
horizontal $hstart_0 $hrow_be $hcols_0 "$char" 

str_dwn_col_0=$hstart_0
str_dwn_row_0=$((hrow_be + 1))
str_dwn 6 $str_dwn_col_0 $str_dwn_row_0 "$char"

hstart_0_lo=$((f_st_cols_2 +4))
hcols_0_lo=6
hrow_0_lo=$((hrow_be+6))
horizontal $hstart_0 $hrow_0_lo $hcols_0 "$char"

str_dwn_col_rig_0=$((hstart_0 +5))
str_dwn_row_rig_0=$((hrow_be + 1))
str_dwn 6 $str_dwn_col_rig_0 $str_dwn_row_rig_0 "$char"
}


one() {
set_color
str_dwn_col_1=$((str_dwn_col_rig_0 + 4))
str_dwn_row_1=$((hrow_be))
str_dwn 7 $str_dwn_col_1 $str_dwn_row_1 "$char"

}



four(){
set_color

start_4_left_col=$((str_dwn_col_1 + 4))
start_4_left_row=$((hrow_be))
str_dwn 3 $start_4_left_col $start_4_left_row "$char"

start_4_hori_col=$((str_dwn_col_1 + 4 ))
start_4_hori_row=$((hrow_be + 3))
horizontal $start_4_hori_col $start_4_hori_row 5 "$char"

start_4_right_col=$((start_4_left_col + 5))
start_4_right_row=$((hrow_be))
str_dwn 7 $start_4_right_col $start_4_right_row "$char"
}


#}

set_color(){
color=$RANDOM 
color=$((color%6))
tput setaf $color
}

cols=`tput cols`
mid=$((cols/2))
start=$((mid-16))
echo "What character do you want to use"
read char


tput clear
rows=5


begin_row=2
num_cols=6

two $start $begin_row $num_cols "$char" $color;zero;one;four

tput setaf 0
tput cup 9 0

Save the script as "2014.sh" and execute it as follows.

$ chmod 777 2014.sh 
$ ./2014.sh 
$ What character do you want to use
*

Christmas bell on linux terminal

Run the following script to generate a bell on the linux terminal.

#!/bin/bash 


str_dwn() {
rows=$1
s_start=$2
char=$4
str_st_row=$3
rows_end=$((str_st_row+rows))
for((i=str_st_row;i<rows_end;i++))
do 
tput cup $i $s_start
printf "$char " 
done
echo""
}


backslash() {
b_st_row=$1
b_st_cols=$2
b_char=$3
b_r_end=$((b_st_row + size ))
for((p=b_st_row;p<b_r_end;p++))
do 
tput cup $p $b_st_cols
printf "$b_char " 
((b_st_cols++))
done
echo""
}



forward() {
f_st_row=$1
f_st_cols=$2
f_char=$3
f_r_end=$((f_st_row + size))

for((i=f_st_row;i<f_r_end;i++))
do 
tput cup $i $f_st_cols
printf "$f_char "
 ((f_st_cols--))
done
echo""

}


horizontal() {
hrow=$2
hstart=$1
hcols=$3
char=$4


end=$((hstart + hcols))

for((i=hstart;i<=end;i++))
do 
tput cup $hrow $i
printf "$char" 
done
echo""
}

gen_rand() {
num1=$1

if [ $num1 -eq 0 ]
then
temp=$(($RANDOM % 2))
rand=$((temp + 2))

else

temp=$(($RANDOM % 5))
rand=$((temp + num1))
fi
}


set_color(){
color=$RANDOM 
color=$((color%6))
tput setaf $color
}

cols=`tput cols`
mid=$((cols/2))
start=$((mid-16))
echo "What character do you want to use"
read char
echo "Choose the size of the bell, enter a number"
read size

tput clear
rows=5


begin_row=2
num_cols=6


set_color

horizontal  $start 5 $size "$char"

forward 5 $start "$char"

back_cols=$((start + $size))
backslash 5 $back_cols "$char"

lower_bar_start_cols=$((start-size))
lower_bar_size=$((size*3))
lower_bar_start_row=$((5 + size))
horizontal  $lower_bar_start_cols $lower_bar_start_row $lower_bar_size "$char"

top_bell_cols=$((start + size/2))

tput cup 4 $top_bell_cols
printf "O"

bottom_bell_cols=$((lower_bar_start_cols + size + size/2))
bottom_bell_row=$((lower_bar_start_row  + 1))
tput cup $bottom_bell_row $bottom_bell_cols
printf "00"

tput setaf 0
tput cup 20 0

Run the script as follows 

$ chmod 777 bell.sh
$ ./bell.sh
 What character do you want to use
*
Choose the size of the bell, enter a number
10

Audio/Sound output coming from both speaker and headphones at same time

On installation of a new version of linux it might happen that the audio/sound output keeps coming from the speaker of the laptop even after plugging in a headphone into its socket. The expected behaviour is that when we plug in the headphone, sound should only be coming in the headphone and nothing should be audible from the laptop speakers.

After going over number of possible solutions using available on the net what worked was updating the alsa driver by downloading the latest one from http://www.alsa-project.org/main/index.php/Download

The default version of driver installed was 1.0.23, but the update one available on the site was 1.0.25.

To solve the above mentioned problem download the latest driver source, compile it and install it using the following commands.

Assume that we have downloaded the driver for version 1.0.25 which has the package name alsa-driver-1.0.25.tar.bz2

$ tar -xjvf alsa-driver-1.0.25.tar.bz2
$ cd alsa-driver-1.0.25
$ ./configure
$ make 
$ sudo make install

Please note that the last step will require root permissions.

Now restart the system and the laptop speaker should get muted automatically on plugging in the headphones.

Creating directory under /proc in kernel 3.10 and above

In the post Creating a read write proc entry in kernel versions above 3.10 we saw the creation of read write proc entry. In the post we will see how to create folder under /proc and then create a entry under it.

To create a directory under /proc we can make use of the function

struct proc_dir_entry *proc_mkdir(const char *name, struct proc_dir_entry *parent);

Where

name: is the name of the directory to be created. parent: parent is the name of directory under which the new directory has to be created.

To create a directory named "newparent" under /proc we will first declare a string having the name of the new folder

char *dirname="newparent"
struct proc_dir_entry *parent

newparent=proc_mkdir(dirname,NULL)

The NULL is passed for parent as we are creating the folder under /proc.

Once the new folder has been created we need to pass the pointer to struct proc_dir_entry to proc_create. The use of proc_create can be seen in the post

proc_create("hello",0,newparent,&proc_fops,NULL)

The full code for the module along with proc creation will be as shown below. Please refer to the post Creating a read write proc entry in kernel versions above 3.10 for more details

proc_dir.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include<linux/sched.h>
#include <asm/uaccess.h>
#include <linux/slab.h>

int len,temp;

char *msg;
char *dirname="newparent";
struct proc_dir_entry *parent;
int read_proc(struct file *filp,char *buf,size_t count,loff_t *offp ) 
{
if(count>temp)
{
count=temp;
}
temp=temp-count;
copy_to_user(buf,msg, count);
if(count==0)
temp=len;
   
return count;
}

int write_proc(struct file *filp,const char *buf,size_t count,loff_t *offp)
{
copy_from_user(msg,buf,count);
len=count;
temp=len;
return count;
}

struct file_operations proc_fops = {
read: read_proc,
write: write_proc
};
void create_new_proc_entry() 
{
parent=proc_mkdir(dirname,NULL);
proc_create("hello",0,parent,&proc_fops);
msg=kmalloc(GFP_KERNEL,10*sizeof(char));
}


int proc_init (void) {
 create_new_proc_entry();
 return 0;
}

void proc_cleanup(void) {
 remove_proc_entry("temp1",NULL);
}

MODULE_LICENSE("GPL"); 
module_init(proc_init);
module_exit(proc_cleanup);

The makefile for the same

ifneq ($(KERNELRELEASE),) 
   obj-m := proc_rw_3_10.o
else 

KERNELDIR ?= /lib/modules/$(shell uname -r)/build 


PWD := $(shell pwd)

default: 
 $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
endif 
clean:
 $(MAKE) -C $(KERNELDIR) M=$(PWD) clean

Compile and load the module using

$ make
$ insmod proc_dir.ko

If there are no errors then we can find the new proc entry under /proc

$ ls /proc/newparent 
hello

Thus we can see that under /proc a new folder called newparent has been created in which the proc entry hello has been created. We can read write to this proc entry like other proc entries

$ echo "hi" > /proc/newparent/hello
$ cat /proc/newparent/hello
hi

Quiz Solution

Answers to Quiz based on chapter-4 of book linux device drivers 3rd ed

1. Which of the following config options need to be enabled to be able to debug kernel.
Ans.CONFIG_DEBUG_KERNEL

2.The first argument passed to printk specifies the
Ans.loglevel for the message to be printed

3.The command that can be used to view the kernel logs from user space is
Ans. dmesg

4. The function create_proc_read_entry returns a pointer to
Ans.struct proc_dir_entry

5.The seq file interface is used for
Ans. Passing kernel information to user space

6. Output of strace shows
Ans. The system calls made by a user application

7. In a kernel oops message EIP points to
Ans. The address of the faulty instruction

8. While using gdb for kernel debugging the kernel core file used passed to gdb is
Ans. /proc/kcore

9.Which of the following keys will launch the kdb
Ans. Pause

10. To use kgdb we need two systems connected by a serial cable
Ans. True

Enabling the traditional Alt+tab function in gnome3

In gnome3 the default bechiaviour of Alt + Tab is different from the traditional gnome behaviour. In gnome3 on pressing alt tab, it displays all the applications that are open, but if more than one window of the same application is open then all the windows are clubbed together and only the application is displayed.

To choose between the windows of the same application we need to wait on the required application or click on it and all the windows of that application gets displayed, and we can click on the window we want to open.

On the other hand the traditional alt tab swithches from one application to next in the order in which they were last used.

If we are not comfortable with this behaviour than we can make gnome perform like the traditions Alt+ tab as shown below.

Click on the activites tab and select the application Advanced Settings by typing "Advanced Settings" which will open a window as shown below.