Basic Linux commands

Useful  Linux Commands:

1. How to find and kill a process in Unix:

find a process: ps -ef |grep tomcat

kill process: kill -9 (pid)

2.How will you figure out free available space in Uni

Ans: df command – Shows the amount of disk space used and available on Linux file systems.du command – Display the amount of disk space used by the specified files and for each subdirectory.
df -h

3. Help for any command

man  command name like man df

4.How will you find files recursively that contains specific words in their contents

Search for a given string in all files recursively

$ grep -r "ramesh" * 

  
5.Search for a given string in a file (case in-sensitive search)

$ grep -i "the" demo_file

6. How will you find files recursively that contains specific words in their filename

find . -maxdepth 1 -name "*string*" -print 

It will find all files in the current directory (delete maxdepth 1 if you want

it recursive) containing "string" and will print it on the screen.

7.How to sort a file using unix command
sort -filename
for numeric sort
sort -n filename

reverse sort
sort -r filename      

reverse sort without duplicate

sort -u -i filename

8. How to count number of lines in a file in unix terminal

wc sort.txt

output : 5  5 41 sort.txt 

The three numbers produced in output correspond to number of lines, 

number of words and number of bytes 

9.How will you list nth column of a flat file in Unix ?
awk '{ print $2 $4 }' filename.txt

10.How will you find a process using specific port in Unix and Windows ?

netstat -tulpn

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      1138/mysqld     
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN      850/portmap        

  
filter for specfic port:
netstat -tulpn | grep :80 

11.How will you copy a file from one Unix host to another Unix host
In the following example, transfer all files (/var/www/html) from remote server called server1 to another server called server2:
scp -r user@server1:/var/www/html/ user@server2:/var/www/html/

I recommend using rsync command which will only push or download updated files. It can copy locally, to/from another host over any remote shell, or to/from a remote rsync daemon. In this example, copy files from remote server called server1 into /backup directory:
rsync -avz -e ssh user@server1:/var/www/html /backup   

Basic Linux/Unix Commands Part1

Command	Explanation
	Commands are entered at terminal Unix is case sensitive ~ Home directory _ and hyphen allowed in file names, spaces are not allowed hidden files start with . There is no recycle bin in Unix. Files deleted cannot be restored. every user has a home directory denoted by ~
passwd	Change password
ls ls –l <directory>ls -l <directory>ls -t ls -a ls -la	List files and subdirectories All information of files or subdirectories /<fullpath> Sort by creation time all directories including hidden directory . – current .. – parent
cd <directory> cd .. cd / cd ~ pwd	Change directory parent directory root directory home directory of user present working directory
man <command>	documentation for all unix commands if there is : at bottom next page of documentation press <spacebar> to get out of documentation press q
cat <filename> more <filename>	displays contents of file, scrolls if file is large display page wise contents of a file
mkdir <directoryname> mv <filename> <destination_directory> cp <source_file> <new_file> cp -R <source_dir> <destination_dir>	create new directory move a file to another location wildcards can be used to move multiple files copy files copy all files and subdirectories recursively
rm <filename> rmdir <directory> rm -R <directory>	There is no recycle bin in unix and file/directory once deleted cannot be recovered remove a file wildcards can also be used remove the directory Empty directory cannot be removed remove everything in directory including the directory
ln <source_file> <destination_file> ln -s <sourc_dir> <destination_dir> ls <linkeddirectory>	Link file Deleting a linked file does not delete original file Link a directory. create a symbolic link
find <wheretofind> -name <whattofind> -print find <wheretofind> -type d -print find <wheretofind> -type f –print find <wheretofind> -size +10M -print	Find a file. Print results on screen Get all directories Get all files Get all files >10MB

Linux Commands Part2

Command	Explanation
history !<line no>	Get all previous commands Run a command from history list
chmod <permission> <filename>	Change file/directory permissions
wc <file_name> wc -w <file_name> wc -l <file_name> ls | wc -l	count of lines, count of words, count of characters count of words count of lines count of files in a directory
head <file_name> tail <file_name> tail -f <file_name>	first few lines last few lines watch last lines of a file as it grows
clear	Clear screen
grep <word> <filename> grep -c <word> <filename> grep -i <word> <filename> grep -v <word> <filename> grep ^<word> <filename> grep <word>$ <filename> grep a. <filename> grep a* <filename> grep a[bc] <filename>	Searh a word in file count of times a word occurs case insensitive search lines which do not have the word line starting with word line ending with word . means any character. word a followed with any character occurrence of a followed with any number of characters searches for words ab or ac in file
diff <filename1> <filename2>	Compare contents of two files
tar –cvf <tar_file_name> <contents> tar –tvf <tar_file_name> tar –xvf <tar_file_name>	Create a tar file Get list of tar file contents Extract contents of tar file
env | more	get list of all environment variables
echo $SHELL echo $PATH	get name of shell get PATH variable contents
which <command>	Get the location of file for command
ps	Get list of all processes
ps aux | grep <username>	Get list of all process running for all users and then sort the processes for a user
kill <processId> kill -9 <processId>	Kills a process Kills a process and all its child processes
vi editor commands
<ctrl>+o <ctrl>+x vi <filename> i <ESC> x :wq :q	Save Exit Open an existing or new file in vi editor Go to insert mode Get out of insert mode Delete a character Write and quit Quit

 

Java Qestions

1. How do you create an Object instance without using new
   1. Using newInstance method of Class class Eg: Class clas = Class.forName("NewClass"); NewClass obj = (NewClass) clas.newInstance(); - Class.forName() loads the class and to create an object, we need to use newInstance() method of Class class. or using class loader: getClass().getClassLoader().loadClass("NewClass").newInstance();
      
   2. Using Object Deserialization Eg : ObjectInputStream objStream = new ObjectInputStream(inputStream );
      NewClass obj = (NewClass ) inStream.readObject();
      
   3. Using clone() of java.lang.Object Eg: NewClass obj = new NewClass();
      NewClass obj2 = obj.clone(); - Creates copy of an existing object