Master Essential Linux Commands: From Login to File Management

Login and Installation Commands

login – Purpose: log into the system (available to all users). Syntax: login [name] [-p] [-h hostname]. Key options: -p preserve the current environment; -h specify a remote host. Example: after the kernel and console information, the prompt shows localhost login:root and then asks for the password.

System Control Commands

shutdown – Purpose: safely shut down the computer (superuser only). Syntax: shutdown [-h] [-i] [-k] [-m] [-t]. Important options: -t set delay before changing runlevel; -k send warning signal only; -h power off after shutdown; -c cancel a pending shutdown; -F force fsck on reboot; -m switch to single‑user mode; -i display system info. Example: shutdown -h 3:40 shuts down at 3:40.

halt – Purpose: halt the system (superuser only). Syntax: halt [-n] [-w] [-d] [-f] [-i] [-p]. Key options: -n avoid writing to /etc/mtab; -w write to /var/log/wtmp only; -d do not leave a record; -f force halt without calling shutdown; -i turn off network interfaces; -p power off after halt.

reboot – Purpose: restart the computer (system administrator). Syntax: reboot [-n] [-w] [-d] [-f] [-i]. Key options: -n skip writing memory to disk; -w write to /var/log/wtmp only; -d do not write to /var/log/wtmp (implied by -n); -i shut down network interfaces before reboot.

File Management Commands

install – Purpose: copy or upgrade files and set permissions (all users). Syntax examples:

install [options] source destination

install [options] source... directory

install -d [options] directory...

Key options: --backup[=CONTROL], -b, -c, -d, --directory, -D, -g, --group=GROUP, -m, --mode=MODE, -o, --owner=OWNER, -p, --preserve‑timestamps, -s, --strip, -S, --suffix=SUFFIX, -v, --verbose, --help, --version.

mount – Purpose: mount a filesystem (superuser or allowed users). Syntax: mount -a [-fv] [-t vfstype] [-n] [-rw] [-F] device dir. Key options: -h show help; -v verbose; -a mount all from /etc/fstab; -F run a helper for each mount; -f simulate mount for debugging; -t vfstype specify filesystem type; -n avoid writing to /etc/mtab. Tips: mount CD‑ROM, floppy, or other devices to directories under /mnt. Example: mount -t iso9660 /dev/hdc /mnt/cdrom.

umount – Purpose: unmount a filesystem (superuser or allowed users). Syntax: umount -a [-fFnrsvw] [-t vfstype] [-n] [-rw] [-F] device dir. It is the reverse of mount; after unmounting a CD‑ROM, the device can be ejected.

chsh – Purpose: change a user's login shell (all users). Syntax: chsh [-s] [-list] [--help] [-v] [username]. Key options: -l list available shells; -v show shell version. Example interaction shows changing from /bin/bash to /bin/tcsh.

exit – Purpose: exit the current shell (all users). Syntax: exit (no parameters).

last – Purpose: display recent login sessions (all users). Syntax:

last [‑n] [‑f file] [‑t tty] [‑h node] [‑i IP] [‑1] [‑y] [‑D] [‑x]

. Key options: -n limit number of records; -f specify log file; -t filter by tty; -h filter by node; -i filter by IP; -1 show IP as remote address; -y show year/month/day; -D specify username; -x show system shutdown, login, and logout history.

file – Purpose: determine file type by examining contents (all users). Syntax: file [options] filename. Key options: -v version; -z check compressed files; -L follow symlinks; -f name read file list from name. Example: file grap reports grap: English text.

mkdir – Purpose: create directories (all users). Syntax: mkdir [options] directory. Key options: -m, --mode=MODE set permissions; -p, --parents create parent directories as needed; -v, --verbose report each created directory; --version show version.

grep – Purpose: search files for patterns (all users). Syntax: grep [options] pattern [file...]. Important options: -c count matching lines; -i ignore case; -h suppress filename in multi‑file output; -l list matching filenames; -n show line numbers; -s suppress error messages; -v invert match. Regex basics are explained (e.g., ^, $, , character classes, ., *).

dd – Purpose: copy and convert files (all users). Syntax: dd [options]. Key options: bs=BYTES, cbs=BYTES, conv=KEYWORDS, count=BLOCKS, ibs=BYTES, if=FILE, obs=BYTES, of=FILE, seek=BLOCKS, skip=BLOCKS. Typical use: creating boot disks, e.g., dd if=vmlinuz of=/dev/fd0.

find – Purpose: locate files in a directory hierarchy (all users). Syntax: find [path] [options] [expression]. Common options: -depth, -maxdepth LEVELS, -mindepth LEVELS, -mount, -version. Expressions: -name PATTERN, -atime N, -ctime N, -group GROUP, -user USER, -size N, -print. Examples: find / -name lilo.conf, find /etc -size +500000c -and -mtime +1.

mv – Purpose: move or rename files/directories (all users). Syntax: mv [options] source target. Key options: -i interactive (prompt before overwrite); -f force (no prompt).

ls – Purpose: list directory contents (all users). Syntax: ls [options] [file]. Important options: -a, --all show hidden files; -A, --almost‑all omit . and ..; -l long format; -F classify file types; -i show inode; -R recursive; -h human‑readable sizes; many others for sorting, formatting, and filtering.

diff – Purpose: compare two files and show differences (all users). Syntax: diff [options] file1 file2. Key options: -a treat all files as text; -b ignore whitespace changes; -B ignore blank lines; -c context output; -H heuristic search; -I ignore case; -n RCS format.

cmp – Purpose: compare two files byte by byte (all users). Syntax: cmp [options] file1 file2. Option: -l output differing byte positions in decimal and octal.

cat – Purpose: concatenate and display file contents (all users). Syntax: cat [options] file1 file2 …. Key options: -n number all output lines; -b number non‑blank lines only; -s squeeze multiple blank lines.

ln – Purpose: create hard or symbolic links (all users). Syntax: ln [options] source [linkname]. Options: -f remove existing destination; -d allow hard links to directories (admin only); -s create symbolic link; -b backup overwritten files. Explanation of hard links (inode sharing) and symbolic links (path reference).

Practical Exercises

1. Run multiple commands in one line using a semicolon, e.g., last -x; halt to view login history then shut down.

2. Mount Windows partitions with mount -t vfat /dev/hda1 /mnt/winc and display Chinese filenames using -o iocharset=cp936.

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 to reach web documents via gg.

5. Use dd to copy a floppy image: dd if=/dev/fd0 of=floppy.fd and load a ramdisk: dd if=root.ram of=/dev/ram0.

6. Use grep in pipelines and with find for advanced searches, e.g., ls -l | grep '^d' to list directories only.

Author: njqyu

Source: blog.51cto.com

《Linux云计算及运维架构师高薪实战班》2018年03月26日即将开课中， 120天冲击Linux运维年薪30万 ，改变速约~~~~

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