Day 5 - Week 3 Review: The Commands on One Page

📌 Key Takeaways

• Read a command as command [options] [arguments], and a path starting with / is absolute.
• Tell apart what needs -r (recursive) and what doesn't — cp -r and rm -r need it, mv doesn't.
• find finds files, grep finds contents. This one sentence separates the two commands.
• /etc/passwd is 7 colon-separated fields, UID 0 = root, and passwords are in /etc/shadow.
• su switches users (target's password), sudo runs one command (your own password) — memorize them as a pair.

The Commands on One Page

  Location      pwd · cd · ls
    │
  Viewing       cat (all) · less (page through) · head (first 10) · tail (last 10) · tail -f (real time)
    │
  Create/delete touch · mkdir (-p) · rmdir (empty only) · rm (-r, -f)
    │
  Copy/move     cp (-r needed) · mv (-r not needed, also renames) · ln (-s symbolic)
    │
  Finding       find (files, path first) · locate (index, fast) · grep (contents)
    │
  Counting/chaining  wc -l · | (pipe)
    │
  Accounts      useradd (-m) · passwd · usermod (-aG) · userdel (-r) · id · su · sudo

Gathering the Meanings of Option Letters

The same letter is used with similar meanings across several commands.

⚠️ The ones marked have different meanings in each command. -f is force in rm but follow in tail. Spots like these show up as trick questions.

Options That Split by Case

Even the same letter, uppercase and lowercase are completely different options. Some questions give four choices that differ only in case.

Practice Reading a Command Line to the End

This week's content comes down to whether you can read a command by breaking it into chunks. Try splitting the lines below from the left.

find /var/log -type f -name "*.log" -mtime -7

• find the command / /var/log the starting path / -type f files only / -name "*.log" whose name ends in .log / -mtime -7 modified within the last 7 days.
• Conditions written side by side give only what satisfies all of them (AND). The - before a number means "more recent than that," and + means "older than that."

grep -v "^#" /etc/fstab | wc -l

• grep -v lines that don't match / "^#" that start with # / /etc/fstab in this file / | pass that result along / wc -l count the lines.
• Read in plain terms, it is "how many lines are not comments?" ^ is the start of a line and $ is the end of a line.

cp -rp /home/user/docs /backup/ && ls -ld /backup/docs

• cp -rp copy the directory through its subdirectories (-r), preserving permissions and times (-p) / && if it succeeds / ls -ld look at the detailed info of the directory itself.
• Since /backup is an existing directory, the result becomes /backup/docs.

sudo useradd -m -g dev -G wheel -s /bin/bash kim

• sudo just this one command with administrator privileges / useradd -m along with a home directory / -g dev the primary group is dev / -G wheel the secondary group is wheel / -s /bin/bash login shell / kim the name of the account to create.
• After running it, a line of the form kim:x:UID:GID of dev::/home/kim:/bin/bash appears in /etc/passwd. Each option fills one of the seven fields.

Files and Directories That Appeared This Week

Separating the "Finding" Commands from the "Asking" Commands

What You Must Be Able to Read Right Off the Screen

[root@www ~]# ls -l /etc/passwd /tmp -d
-rw-r--r--.  1 root root 2104 Sep  3 09:00 /etc/passwd
drwxrwxrwt. 12 root root 4096 Sep  3 09:10 /tmp

Contrasting Easily Confused Pairs

Commonly Missed Points

1. You have to add -r to mv → No. mv simply moves directories too. -r is needed for cp and rm.
2. find -name "*.txt" /home → The path comes first. find /home -name "*.txt".
3. Adding a group with usermod -G → It wipes out the existing secondary groups. You must use -aG.
4. UID 1 is root → It is 0.
5. Deleting a non-empty directory with rmdir → It doesn't work. You must use rm -r.
6. The password is in /etc/passwd → It is in /etc/shadow. The x in passwd is the marker for that.
7. Your own password for su → It is the target account's password. Using your own is sudo.
8. Hidden files are a separate attribute → No. Their name just starts with ..

Values That Appear on the Exam Exactly

Next week is about permissions attached to files — we will cover rwx and octal, ownership, and special permissions.

📖 Glossary

• Recursive : The action of walking through subdirectories. The -r/-R options.
• Pager : A program that breaks long output into screen-sized pieces. less, more.
• Pipe (|) : A symbol that passes the output of the previous command as the input of the next command.
• UID / GID : Numbers that identify users and groups. UID 0 is root.
• System account : An account for running daemons. Its login shell is usually /sbin/nologin.
• Principle of least privilege : The security principle of giving only as much privilege as needed. It is the reason sudo is recommended.