Wiley Edge Linux Workshop

1. Print the 10th line of the file.
   head -n10 file.txt | tail -n1

• Note: Head gets the top n number of lines, tail gets the last n number of lines. So we can get the last line after piping to tail

2. Write a script that prints any given file line number.

• The first arg should be the file name
• Second arg the line number
• When complete, we will review a suggested solution
  • How can we check if it ran successfully with the suggested solution?
  • echo $? will give us the exit code of the previous command. 0 is a pass. Anything else is fail

3. In Unix, what are inodes (index nodes)?

• Unix systems have a database where every file name points to an inode number
• The inode number often is the disk location of the inode, but the inode itself is a struct (like in C ) that contains information about each file. The information may include file size, permissions, group, owner, and blocks.
• The blocks are the actual memory locations the files are stored on, similar to arrays, except the blocks do not have to be in order. When files are stored in blocks that are in non-sequential memory locations, this is called fragmentation.

4. Suppose in an interview you were asked to list the inode number of a file. Use the man command on ls to find out how to print inode keys. Search for the word inode in the manual. Finding an answer fast in an interview will earn you bonus points for problem-solving and resourcefulness.
   ls -i

5. Print the inode information of any file in the home folder.
   stat file.txt

6. Start the logger.sh script. Send it to the background (ctrl +z), start it in the background (bg 1), and send it to the foreground again (fg 1). Use the jobs command in between each step. Repeat if you did not use jobs in between each step.

7. Kill the process (ctrl +c if in the foreground, or kill -9 PID if in the background) and start it again with & so that it starts in the background

8. Confirm it is running with ps and grep
   ps -eaf | grep logger | grep -v grep

• Note: The second grep removes the first grep searching for the actual process!
• Note: Run ps -eaf without grep. Notice the parent process to the logger script is the current bash session. If you exit bash, all processes whose parent is bash will be terminated as well!

9. Now save the PID in a variable named PID PID=$(ps -eaf | grep logger | grep -v grep | awk '{print $2')

• what is awk doing here?

10. What files are the process writing?
    lsof -p $PID | more.
    or, for live view
    sudo strace -f -t -e trace=file -p $PID.
    You can even grep for the file descriptors to get files being appended.
    sudo strace -f -t -e trace=file -p 42 2>&1 | egrep "O_WRONLY\|O_CREAT\|O_APPEND".

• Look up these file descriptors to understand what they mean

11. Exit your terminal session, log back in, and look for the process. Is it still there? How can we make it persist?

• You can use nohub or screen. Practice doing this. It is important for commands that need to be executed longer than your bash session (especially if you are having connectivity or timeout issues)
• If using screen, just enter screen, start your process, detatch with ctrl + a then d. Read the manual for screen and find out how to list screen sessions and attach to one.
• Kill your screen session with screen -Xs <sessionid> quit, or enter screen session and type exit

Conclusion

• We started with a simple script that helped us review best bash practices (args, defensive coding)
• We reviewed how to quickly use the manual to find answers you may be asked in interviews. Interviewers love to see your problem-solving approach, and searching for text in a manual is an excellent skill in that situation.
• Ran a simple script in the background and found information about that process, including which files it has open.
• Exited the terminal and demonstrated that any child process of the bash process would be terminated. To keep this process running, we used nohub or screen. Be careful to keep child processes running only as long as intended.
• If you ever need to kill all child processes, run kill $(ps -o pid= --ppid $$), where $$ is the current bash process and can be replaced with any PID