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Sunday, 6 September 2015

Linux One Liners #1

1. How to display the 10th line of a file?
head -10 filename | tail -1

2. How to remove the header from a file?
sed -i '1 d' filename

3. How to remove the footer from a file?
sed -i '$ d' filename

4. Write a command to find the length of a line in a file?
The below command can be used to get a line from a file.
sed –n '<n> p' filename
We will see how to find the length of 10th line in a file
sed -n '10 p' filename|wc -c

5. How to get the nth word of a line in Unix?
cut –f<n> -d' '

6. How to reverse a string in unix?
echo "java" | rev

7. How to get the last word from a line in Unix file?
echo "unix is good" | rev | cut -f1 -d' ' | rev

8. How to replace the n-th line in a file with a new line in Unix?
sed -i'' '10 d' filename # d stands for delete
sed -i'' '10 i new inserted line' filename # i stands for insert

9. How to check if the last command was successful in Unix?
echo $?

10. Write command to list all the links from a directory?
ls -lrt | grep "^l"

11. How will you find which operating system your system is running on in UNIX?
uname -a

12. Create a read-only file in your home directory?
touch file; chmod 400 file

13. How do you see command line history in UNIX?
The 'history' command can be used to get the list of commands that we are executed.

14. How to display the first 20 lines of a file?
By default, the head command displays the first 10 lines from a file. If we change the option of head, then we can display as many lines as we want.
head -20 filename
An alternative solution is using the sed command
sed '21,$ d' filename
The d option here deletes the lines from 21 to the end of the file

15. Write a command to print the last line of a file?
The tail command can be used to display the last lines from a file.
tail -1 filename
Alternative solutions are:
sed -n '$ p' filename
awk 'END{print $0}' filename

16. How do you rename the files in a directory with _new as suffix?
ls -lrt|grep '^-'| awk '{print "mv "$9" "$9".new"}' | sh

17. Write a command to convert a string from lower case to upper case?
echo "apple" | tr [a-z] [A-Z]

18. Write a command to convert a string to Initcap.
echo apple | awk '{print toupper(substr($1,1,1)) tolower(substr($1,2))}'

19. Write a command to redirect the output of date command to multiple files?
The tee command writes the output to multiple files and also displays the output on the terminal.
date | tee -a file1 file2 file3

20. How do you list the hidden files in current directory?
ls -a | grep '^\.'

21. List out some of the Hot Keys available in bash shell?
Ctrl+l - Clears the Screen.
Ctrl+r - Does a search in previously given commands in shell.
Ctrl+u - Clears the typing before the hotkey.
Ctrl+a - Places cursor at the beginning of the command at shell.
Ctrl+e - Places cursor at the end of the command at shell.
Ctrl+d - Kills the shell.
Ctrl+z - Places the currently running process into background.

22. How do you make an existing file empty?
cat /dev/null > filename

23. How do you remove the first number on 10th line in file?
sed '10 s/[0-9][0-9]*//' < filename

24. What is the difference between join -v and join -a?
join -v : outputs only matched lines between two files.
join -a : In addition to the matched lines, this will output unmatched lines also.

25. How do you display from the 5th character to the end of the line from a file?
cut -c 5- filename

26. Display all the files in current directory sorted by size?
ls -l | grep '^-' | awk '{print $5,$9}' |sort -n|awk '{print $2}'

27. Write a command to search for the file 'map' in the current directory?
find -name map -type f

28. How to display the first 10 characters from each line of a file?
cut -c -10 filename

29. Write a command to remove the first number on all lines that start with "@"?
sed '\,^@, s/[0-9][0-9]*//' < filename

30. How to print the file names in a directory that has the word "term"?
grep -l term *
The '-l' option make the grep command to print only the filename without printing the content of the file. As soon as the grep command finds the pattern in a file, it prints the pattern and stops searching other lines in the file.

31. How to run awk command specified in a file?
awk -f filename

32. How do you display the calendar for the month march in the year 1985?
The cal command can be used to display the current month calendar. You can pass the month and year as arguments to display the required year, month combination calendar.
cal 03 1985
This will display the calendar for the March month and year 1985.

33. Write a command to find the total number of lines in a file?
wc -l filename
Other ways to pring the total number of lines are
awk 'BEGIN {sum=0} {sum=sum+1} END {print sum}' filename
awk 'END{print NR}' filename

34. How to duplicate empty lines in a file?
sed '/^$/ p' < filename

35. Explain iostatvmstat and netstat?
Iostat: reports on terminal, disk and tape I/O activity.
Vmstat: reports on virtual memory statistics for processes, disk, tape and CPU activity.
Netstat: reports on the contents of network data structures.

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