Essential Linux Commands: Installation and Login Basics Explained

Introduction

Linux provides a large set of commands that allow you to perform disk operations, file access, directory manipulation, process management, and permission settings; mastering these commands is essential for understanding the Linux system.

Command Categories

The most important and frequently used commands are divided into six groups:

Installation and login commands

File handling commands

System management commands

Network operation commands

System security commands

Other useful commands

Installation and Login Commands

login

Purpose: log in to the system (available to all users).

Syntax: login [name] [-p] [-h hostname] Key options: -p: preserve the current environment. -h: specify the remote host name.

Example:

Mandrake Linux release 9.1 (Bamboo) for i586
renrel 2.4.21-0.13mdk on i686 / tty1
localhost login:root
password:

After entering the username and password you are logged in; the password is not echoed for security.

shutdown

Purpose: safely shut down the computer (super‑user only).

Syntax: shutdown [-h] [-i] [-k] [-m] [-t] Key options: -t: time before changing runlevel. -k: send warning signal only, do not actually shut down. -h: halt and power off after shutdown. -c: cancel a pending shutdown. -F: force fsck on reboot. -i: display system information during shutdown.

Example: # shutdown -h now halt

Purpose: halt the system (super‑user only).

Syntax: halt [-n] [-w] [-d] [-f] [-i] [-p] Key options: -n: prevent sync system call. -w: write wtmp record only. -f: force halt without calling shutdown. -i: bring down network interfaces before halt. -p: power off after halt. -d: halt without leaving a record.

Example: # halt -p reboot

Purpose: restart the computer (system administrator only).

Syntax: reboot [-n] [-w] [-d] [-f] [-i] Key options: -n: do not write memory to disk before reboot. -w: write to wtmp only. -d: do not write to wtmp (implied by -n). -i: stop network interfaces before reboot.

Example: # reboot -f install

Purpose: copy files, set permissions, or create directories (all users).

Syntax:

install [options] source destination
install [options] source... directory
install -d [options] directory...

Key options: --backup[=CONTROL]: backup existing destination files. -b: backup without arguments. -c: (no effect). -d, --directory: treat arguments as directories and create parents. -D: create missing parent directories before copying. -g, --group=GROUP: set group ownership. -m, --mode=MODE: set permission bits. -o, --owner=OWNER: set file owner (super‑user only). -p, --preserve-timestamps: preserve access/modification times. -s, --strip: strip symbols (for binaries). -S, --suffix=SUFFIX: set backup suffix. -v, --verbose: show each file processed. --help, --version mount

Purpose: mount a filesystem (super‑user or users allowed in /etc/fstab).

Syntax: mount [-a] [-fv] [-t vfstype] [-n] [-rw] [-F] device dir Key options: -a: mount all filesystems listed in /etc/fstab. -F: fork a helper process for each mount (useful for many NFS mounts). -f: fake mount (debug mode). -v: verbose output. -t vfstype: specify filesystem type. -n: do not write to /etc/mtab.

Example (mount a Windows FAT partition): # mount -t vfat /dev/hda1 /mnt/winc To display Chinese filenames on the mounted partition:

# mount -t vfat -o iocharset=cp936 /dev/hda1 /mnt/winc

umount

Purpose: unmount a filesystem (super‑user or allowed users).

Syntax:

umount [-a] [-fFnrsvw] [-t vfstype] [-n] [-rw] [-F] device dir

Example: # umount /cdrom chsh

Purpose: change a user's login shell (all users).

Syntax: chsh [-s shell] [-list] [--help] [-v] [username] Key options: -l: list all available shells. -v: show shell version.

Example:

Changing login shell for cao
Password:
New shell [/bin/bash]: /bin/tcsh

exit

Purpose: exit the current shell (all users).

Syntax: exit last

Purpose: show recent login sessions (all users).

Syntax: last [-n] [-f file] [-t tty] [-h] [-I IP] [-1] [-y] [ID] Key options: -n: limit number of records. -f file: specify log file. -t tty: show records for a specific virtual console. -h: filter by hostname. -I IP: filter by IP address. -1: display IP instead of hostname. -y: show year, month, day. -ID: filter by username. -x: show system shutdown, login and logout history.

File Handling Commands

file

Purpose: determine file type (all users).

Syntax: file [options] filename Key options: -v: show version. -z: test compressed files. -L: follow symlinks. -f name: read file names from name.

Example:

$ file grap
grap: English text

mkdir

Purpose: create a directory (all users).

Syntax: mkdir [options] dirname Key options: -m, --mode=MODE: set permission bits. -p, --parents: create parent directories as needed. -v, --verbose: report each created directory.

Example: $ mkdir -m 777 tsk grep

Purpose: search for patterns in files (all users).

Syntax: grep [options] pattern [file...] Key options: -c: count matching lines. -i: ignore case. -h: suppress filename prefix. -l: list only matching filenames. -n: show line numbers. -s: suppress error messages. -v: invert match.

Example (find IP‑like strings): $ grep '[0-9]\{3\}\.[0-9]\{3\}' ipfile dd

Purpose: convert and copy files at low level (all users).

Syntax: dd [options] Key options: bs=BYTES: set both input and output block size. cbs=BYTES: convert BYTES at a time. conv=KEYWORDS: conversion flags. count=BLOCKS: copy only BLOCKS input blocks. ibs=BYTES: input block size. if=FILE: input file. obs=BYTES: output block size. of=FILE: output file. seek=BLOCKS: skip BLOCKS on output. skip=BLOCKS: skip BLOCKS on input.

Example (write a Linux kernel to a floppy): # dd if=vmlinuz of=/dev/fd0 find

Purpose: search for files in a directory hierarchy (all users).

Syntax: find [path] [options] [expression] Key options: -depth: process contents before the directory itself. -maxdepth N: descend at most N levels. -mindepth N: descend at least N levels. -mount: stay on the same filesystem. -name PATTERN: match filename (supports * and ?). -atime N: files accessed N days ago. -ctime N: files changed N days ago. -user USER: files owned by USER. -group GROUP: files belonging to GROUP. -size N: files of size N blocks. -print: output the found path.

Example (find a file named lilo.conf on the whole system): # find / -name lilo.conf mv

Purpose: move or rename files and directories (all users).

Syntax: mv [options] source target Key options: -i: interactive, ask before overwriting. -f: force, do not ask.

Example (rename a file): $ mv cjh.txt wjz.txt ls

Purpose: list directory contents (all users).

Syntax: ls [options] [file...] Key options (selected): -a, --all: do not ignore entries starting with . -l: long format. -h: human‑readable sizes. -R, --recursive: list subdirectories recursively. -S: sort by size. -t: sort by modification time. -r: reverse order.

Example (long listing): $ ls -l diff

Purpose: compare two files line by line (all users).

Syntax: diff [options] file1 file2 Key options: -a: treat all files as text. -b: ignore changes in the amount of whitespace. -B: ignore blank lines. -c: produce context diff. -H: speed up search on large files. -I: ignore case. -n, --rcs: RCS format.

cmp

Purpose: compare two files byte by byte (all users).

Syntax: cmp [options] file1 file2 Key option: -l: output differing byte numbers in octal and decimal.

cat

Purpose: concatenate and display files (all users).

Syntax: cat [options] file1 [file2 ...] Key options: -n: number all output lines. -b: number non‑blank lines. -s: squeeze multiple blank lines.

Example (display a README file): $ cat README ln

Purpose: create hard or symbolic links (all users).

Syntax: ln [options] source [linkname] Key options: -s: create a symbolic link. -f: remove existing destination files. -d: allow hard‑linking directories (super‑user). -b: backup existing files before linking.

Example (create a shortcut to a web‑server document root):

$ ln -s /usr/local/httpd/htdocs gg

System Management Commands

df

Purpose: report filesystem disk space usage (all users).

Syntax: df [options] Key options: -h: human‑readable sizes. -T: show filesystem type. -i: display inode usage. -k: show sizes in kilobytes. -a: include pseudo, duplicate, and inaccessible filesystems.

Example:

# df -T
Filesystem   Type   1K-blocks   Used   Available   Use%   Mounted on
/dev/hda7    reiserfs   5.2G   1.6G   3.7G   30%   /
/dev/hda1    vfat       2.4G   1.6G   827M   66%   /windows/C

top

Purpose: display running processes and system load (all users).

Syntax: top [-] [-d delay] [-q] [-c] [-S] [-s] [-i] [-n] Key options: -d: update interval in seconds. -q: no delay, run with highest priority if super‑user. -c: show full command line. -S: cumulative mode. -i: hide idle and zombie processes. -n: number of updates before exiting.

Interactive keys:

Space: immediate refresh.

P: sort by CPU usage.

M: sort by memory usage.

T: sort by time.

c: toggle command line/full path.

q: quit.

free

Purpose: display memory usage (all users).

Syntax: free [options] Key options: -b, -k, -m: display in bytes, kilobytes, megabytes. -s SECONDS: repeat every SECONDS. -t: show total line. -o: omit the buffer/cache line.

Example (continuous monitoring every 5 seconds): # free -b -s5 quota

Purpose: display disk usage limits (super‑user).

Syntax: quota [-g] [-u] [-v] [-p] user Key options: -g: show group limits. -u: show user limits. -v: show limits for filesystems without quotas. -p: display a concise report.

Example:

# quota caojh
Disk quotas for user caojh (uid 502):
Filesystem  blocks  quota  limit  grace  files  quota  limit  grace
/dev/hda3   58    200000 400000  41    500   1000

at

Purpose: schedule a command to run once at a specified time (all users).

Syntax: at [options] time Key options: -f file: read commands from file. -m: mail you when the job finishes. -q queue: select a queue. -V: verbose.

Example (run data two days later at 15:30): # at -f data 15:30 +2 days lp

Purpose: print files (all users).

Syntax: lp [options] file... Key options: -c: copy before printing. -d: specify printer queue. -m: mail when done. -n: number of copies. -p: set priority (0‑100, default 50).

Example: # lp -d LaserJet -p 90 /etc/aliases useradd

Purpose: create a new user account (super‑user).

Syntax: useradd [options] username Key options: -c comment: GECOS field. -d home: home directory. -s shell: login shell. -m: create home directory. -u UID: specify user ID. -g GID: primary group. -G groups: supplementary groups. -e date: account expiration date. -f days: days after password expiry before disabling. -r: create a system account.

Example: # useradd caojh -u 544 groupadd

Purpose: add a new group to the system (super‑user).

Syntax: groupadd [options] groupname Key options: -g GID: specify group ID. -o: allow non‑unique GID. -r: create a system group (GID < 500). -f: exit silently if the group already exists.

Example: # groupadd -g 344 cjh kill

Purpose: send a signal to a process (all users).

Syntax: kill [-s signal | -p] [-a] pid... Key options: -s signal: specify the signal to send. -l: list signal names. -p: simulate sending a signal.

Example (force kill PID 324): # kill -9 324 Kill can also be used to reload configuration files, e.g. after editing /etc/inetd.conf:

# ps -ef | grep inetd   # find PID, assume 1426
# kill -HUP 1426          # make inetd reread its config

crontab

Purpose: maintain a table of periodic commands (all users).

Syntax: crontab [options] file or crontab -e|-l|-r Key options: -e: edit the current crontab with the default editor. -l: list the current crontab. -r: remove the current crontab. -u user: operate on another user's crontab (super‑user).

Crontab entry format: M H D m d command (minute, hour, day of month, month, day of week).

Example (run antivir every day at 09:05): 05 09 * * * antivir After saving, the job will be stored in /var/spool/cron/username.

Practice Exercises

1. Combine multiple commands on one line using a semicolon, e.g. # last -x; halt to display login history and then shut down.

2. Mount a Windows partition on /mnt/winc: # mount -t vfat /dev/hda1 /mnt/winc To display Chinese filenames:

# mount -t vfat -o iocharset=cp936 /dev/hda1 /mnt/winc

3. Mount a USB flash drive: # mount /dev/sda1 /usb 4. Create symbolic links for quick access, e.g. # ln -s /usr/local/httpd/htdocs gg.

5. Use dd to write a kernel image to a floppy: # dd if=vmlinuz of=/dev/fd0.

6. Use grep together with ls to list only directories: $ ls -l | grep '^d'.

7. Use kill together with top to observe system performance changes after terminating a process.

8. Schedule a shutdown with at:

# at 17:35
at> halt -i -p
at> ^D

9. Use crontab to schedule a daily virus scan at 09:05.

10. After editing /etc/inetd.conf, reload it with kill -HUP $(pidof inetd).