Program Components

A typical program consists of:

• Binary executables
• Shared libraries (non-executable on their own, only loaded when called)
• Configuration files
• Documentation or help files

Command Structure

The general syntax is: COMMAND [OPTIONS] [ARGUMENTS]

Command Execution

Commands are executable binaries that may depend on shared libraries. Common locations include:

• User commands: /bin, /usr/bin, /usr/local/bin
• Administrative commands: /sbin, /usr/sbin, /usr/local/sbin
• Libraries (32-bit): /lib, /usr/lib, /usr/local/lib
• Libraries (64-bit): /lib64, /usr/lib64, /usr/local/lib64

Not all commands correspond to a file in these directories—some are shell built-ins.

The PATH environment variable defines the search order for command resolution (left to right). View it with:

echo $PATH

Options

• Short form: -l, -d; combinable as -ld
• Long form: --help, --human-readable; not combinable
• Some options accept parameters (e.g., -t 1909131212)

Arguments

Specify targets of the command (e.g., files or directories). Multiple arguments are space-separated:

ls -ld /var /etc

Accessing Help

• Built-in commands: help COMMAND
• External commands:
  • COMMAND --help
  • Manual pages: man COMMAND

Manual sections:

1. User commands
2. System calls
3. Library functions
4. Special files
5. File formats
6. Administration tools

Within man, navigation includes:

• Space: next page
• b: previous page
• /keyword: search forward
• ?keyword: search backward
• q: quit

Core Commands

Directory Navigation

• cd: home directory
• cd ~user: user’s home
• cd -: toggle between last and current directory

Relevant variables:

• $PWD: current directory
• $OLDPWD: previous directory

Listing Files (ls)

Common flags:

• -a: show hidden files
• -l: detailed listing
• -h: human-readable sizes
• -d: list directory itself
• -R: recursive listing

Example output:

-rw-r--r--. 1 root root 8957 Oct 14 19:34 boot.log

Breakdown:

• -: regular file (d = directory, l = symlink, etc.)
• rw-r--r--: permissions for owner/group/others
• 1: hard link count
• root root: owner and group
• 8957: size in bytes
• Timestamp and filename

File Viewing

• cat file: output entire file
  • -n: line numbers
  • -E: show $ at line ends
• tac file: reverse line order
• head -n 10 file: first 10 lines
• tail -n 10 file: last 10 lines
• less file: interactive paging (preferred over more)

Echo and Quoting

echo -e "line1\nline2"

• -e: interpret escape sequences
• Single quotes '...': literal (no variable expansion)
• Double quotes "...": allow variable substitution
• ${VAR}: explicit variable reference

System Control

• Shutdown: shutdown -h now
• Reboot: shutdown -r +5 "Update"
• Cancel: shutdown -c

Time Management

• Hardware clock ↔ system clock:
  • hwclock --hctosys (sync system from hardware)
  • hwclock --systohc (sync hardware from system)

Command Location and Aliasing

• which ls: shows full path of executable
• whereis bash: locates binary, source, and manual
• Aliases (session-only):

  alias ll='ls -l'
  unalias ll
  

Filesystem Hierarchy Standard (FHS)

Key directories:

• /bin, /sbin: essential binaries
• /boot: kernel and bootloader files
• /dev: device files (block/character)
• /etc: static configuration
• /home: user home directories
• /root: admin home
• /lib, /lib64: shared libraries
• /media, /mnt: mount points
• /opt: optional software
• /tmp: temporary files (world-writable)
• /usr: read-only shared data (bin, lib, share, local, etc.)
• /var: variable data (logs, spools, caches)
• /proc, /sys: virtual filesystems exposing kernel/process info

File Types

• -: regular file
• d: directory
• b: block device
• c: character device
• l: symbolic link
• p: named pipe
• s: socket

Devices use major/minor numbers to identify type and instance.

Bash Features

Command History

• history: list recent commands
• !5: re-run command #5
• !ssh: re-run last command starting with "ssh"
• Esc + .: insert last argument of previous command

Brace Expansion

• mkdir -p /tmp/x/{y1/{a,b},y2} → creates nested dirs
• touch {a,b}_{x,y}.txt → a_x.txt, a_y.txt, b_x.txt, b_y.txt

Exit Status

• Success: 0
• Failure: 1–255
• Check with: echo $?

Keyboard Shortcuts

• Ctrl+a: beginning of line
• Ctrl+e: end of line
• Ctrl+u: delete to start
• Ctrl+k: delete to end
• Ctrl+l: clear screen

File Operations

Metadata Inspection

• stat file: shows access (atime), modify (mtime), and change (ctime) timestamps

Timestamp Modification

• touch -a file: update access time
• touch -m file: update modify time
• touch -t [[CC]YY]MMDDhhmm[.ss] file

Copying (cp)

• Single file: cp src dest
• Recursive: cp -r dir1 dir2
• Archive mode (preserves all attributes): cp -a src dest
• Interactive: cp -i src dest

Moving/Renaming (mv)

• mv old new
• Supports -i (prompt) and -f (force)

Deletion (rm)

• Files: rm file
• Directories: rm -r dir
• Forceful removal: rm -rf dir

⚠️ Caution: rm -rf /* is catastrophic. Prefer moving files to a trash directory instead of immediate deletion.