Day 5 - Week 6 Comprehensive Review: Editors and Processes

📌 Key Points

• Only vi has modes. Command → i → insert, ESC → command, : → ex.
• :%s/A/B/g — % is the whole file, g is everything within a line.
• ps is one scene, top is real time. ps -ef (System V) / ps aux (BSD).
• Signal 9 is forced, 15 is the default. For nice, -20 is the highest.
• cron repeats (5 fields), at runs once.

Key Points Diagram

   Editors
     Line editors     ed · ex · sed
     Screen editors   vi(Bill Joy) · emacs(Stallman) · nano(pico improvement)
     Only vi has modes
 
     vi modes    command ──i a o──► insert ──ESC──► command ──:──► ex
     Save/quit   :w  :q  :wq  :q!(force)  ZZ
     Substitute  :%s/A/B/g        % whole file · g everything in a line
     Settings    ~/.vimrc (personal) · /etc/vimrc (global)
 
   Processes
     PID 1       init → systemd     ancestor of all processes
     Orphan      parent died first → PID 1 adopts it
     Zombie(Z)   ended but parent did not collect → disappears only when the parent is cleaned up
 
     Foreground ──Ctrl+Z──► stopped ──bg──► background
          ▲                                  │
          └──────────── fg ──────────────────┘
 
     Signals     1 HUP · 2 INT(Ctrl+C) · 9 KILL(forced) · 15 TERM(default)
     Priority    -20 ◄─ high ── 0 ── low ─► 19
 
   Scheduling
     cron  repeat  minute hour day month weekday   crontab -e
     at    once    at 22:00

Revisiting It Through Scenes

vi — Editing One Line of a Configuration File

Follow which mode you are in each time you press a key.

   What you press         What happens                              Mode
   ─────────────────────────────────────────────────────────────────────
   vi /etc/hosts          The file opens                            command
   /server1  Enter        Move to the line containing server1       command
   A                      Go to the end of that line and start input insert
    web1                  Characters are entered                    insert
   ESC                    Finish input                              command
   yy  p                  Copy that line and paste it below         command
   :s/web1/web2/  Enter   Change web1 to web2 on the pasted line    ex → command
   :set nu  Enter         Turn on line numbers to check             ex → command
   :wq  Enter             Save and quit                             ex

• The only stretch spent in insert mode is from A to ESC. Everything else starts from command mode.
• After p, the cursor is on the newly pasted line. That is why :s without a range changes only that line.
• Search (/) and ex commands (:) are both entered on the bottom line of the screen, and when you press Enter you return to command mode.

Processes — Cleaning Up an Unresponsive Program

ps -ef | grep myapp          # ① check the PID and PPID
# user     3100    2314  5 10:10 pts/0    00:05:12 ./myapp
kill 3100                    # ② signal 15 (TERM) first — give it a chance to clean up
ps -p 3100                   # ③ see whether it is still there
kill -9 3100                 # ④ if it remains, signal 9 (KILL)

• 15 first, 9 if that does not work. If you use 9 from the start, data that was being saved can be corrupted.
• If the state in ① was Z (<defunct>), ② and ④ are useless. You must look at the PPID (here, 2314) side.
• To end everything by name at once, use killall myapp; to have it merely yield instead of stopping it, use renice 10 -p 3100.

crontab Reading Practice

• Point at minute, hour, day, month, weekday one field at a time from the left. A field with a number means "only then," and * means "every time."
• Look first at whether the minute field is * or 0. Most traps are here.

Which Command for Which Situation

Contrasting Easily Confused Pairs

Commonly Missed Points

1. Opening vi lets you type right away → No. It starts in command mode.
2. :q can always exit → It is refused if you made edits. Use :q!.
3. :s/A/B/ replaces everything → It replaces only the first one on that line. For the whole file use :%s/A/B/g.
4. emacs and nano also have modes → They do not. Only the vi family has modes.
5. ps -aux → The BSD style is ps aux without a hyphen.
6. kill defaults to 9 → It is 15 (TERM). 9 must be specified explicitly.
7. Kill a zombie with kill -9 → It is already dead, so that is useless. You must clean up the parent.
8. The larger the nice value, the higher the priority → It is the opposite. -20 is the highest.
9. The first crontab field is hour → It is minute. The order is minute, hour, day, month, weekday.
10. at runs repeatedly → Only once. Repeating is cron.
11. ZZ is an ex-mode command → You press it without a colon in command mode.
12. N always searches upward → It is the opposite of the direction first searched. If you searched with ?, it goes downward.
13. Pressing r enters insert mode → It changes one character and stays in command mode.
14. The program ended with Ctrl+Z → It was only stopped. It remains in jobs as Stopped.
15. Adding only nohup sends it to the background → No. You must use & together.
16. The numbers for nice and renice mean the same thing → nice is the amount to add, and renice is the value itself.
17. You give a PID to killall → You give it a process name. PIDs go with kill.
18. crontab -r deletes one line → It deletes everything. To delete just one line, open with -e and edit.
19. An orphan process is a dead process → It is alive and running. The dead one is the zombie.
20. disable = no turns the service off → The opposite. It means enabled.

Values That Appear on the Exam as They Are

Files at a Glance

Save and Quit Keys at a Glance

Next week is software and devices. We cover package management such as RPM, YUM, DEB, and APT, plus printers, sound, and scanners.

📖 Glossary

• Mode : The state by which vi distinguishes input from commands. There are three: command, insert, and ex.
• Zombie process : A process that has terminated but remains in the table because the parent has not collected it.
• Signal : A notification sent to a process. Delivered with kill, and the default is 15.
• nice value : Process priority. The smaller, the higher, and the range is -20 to 19.
• cron / at : A recurring scheduling tool / a one-time scheduling tool.
• standalone / inetd : The method of keeping a daemon resident / the method of launching it on request.