Linux: basic command

2026-09-02

1. Shell basics

Command	What it does
`chsh`	Change your login shell (prompts if no shell is given)
`pwd`	Print current working directory
`file <name>`	Identify the type of a file
`less <file>`	Page through a file (`q` to quit, `/` to search)
`head -n N` / `tail -n N`	Show first / last N lines
`grep -i -v`	Search text: `-i` ignore case, `-v` invert match
`diff a b`	Show differences between two files
`find <dir> -name <file> -print`	Search directory tree by name
`locate <name>`	Fast search using a prebuilt index
`man -k <keyword>`	Search manual pages by keyword
`passwd`	Change your password

Environment variables

VARIABLE=value        # shell variable (current shell only)
export VARIABLE       # make it an environment variable (visible to child processes)

$PATH — colon-separated list of directories the shell searches when locating a command.

2. Special symbols

Symbol	Name	Meaning in Linux
`Esc`	Escape	Leaves modes (e.g. insert mode in vi/vim)
`~`	Tilde	Home directory: `cd ~`
`	Backtick	Command substitution (old style): `date`
`!`	Bang	History: `!!` last command, `!$` its last argument
`@`	At	`user@hostname` in prompts and SSH
`#`	Hash	Comment marker; root’s prompt symbol
`$`	Dollar	Variables (`$HOME`, `$1`); regular user’s prompt
`%`	Percent	Job control (`%1`); modulo in arithmetic
`^`	Caret	Start of line in regex: `grep '^root' /etc/passwd`
`&`	Ampersand	Run in background: `./script.sh &`
`*`	Star	Wildcard, matches anything: `rm *.log` (dangerous!)
`( )`	Parentheses	Grouping / subshells
`_`	Underscore	No special meaning — safe in filenames
`-`	Dash	Command options: `ls -l`
`{ }`	Curly braces	Brace expansion: `mkdir dir{1,2,3}`
`[ ]`	Square brackets	Character sets (`file[12]`); test conditions in scripts
`:`	Colon	vi commands (`:wq`); separator in `$PATH`, `/etc/passwd`
`;`	Semicolon	Chain commands: `cd /tmp; ls`
`" "`	Double quotes	One argument, variables still expand
`' '`	Single quotes	One argument, nothing expands
`\|`	Pipe	Send output of one command to input of another
`\`	Backslash	Escape next character (Windows path separator, not Linux)
`<`	Less-than	Input redirection
`>`	Greater-than	Output redirection (overwrites)
`>>`	Double greater-than	Output redirection (appends)
`.` `..`	Dot, dot-dot	Current / parent directory; leading dot hides a file
`?`	Question mark	Wildcard for exactly one character: `file?.txt`
`/`	Forward slash	Path separator and filesystem root

3. Redirection and pipes

File descriptors: 0 = stdin, 1 = stdout, 2 = stderr.

cmd > file        # stdout to file (overwrite)
cmd >> file       # stdout to file (append)
cmd 2> file       # stderr to file
cmd > file 2>&1   # stdout and stderr to the same file
cmd < file        # file as stdin
cmd1 | cmd2       # stdout of cmd1 -> stdin of cmd2

4. Processes

ps x          # all your processes
ps ax         # all processes on the system
ps u          # more detailed information
ps w          # full command names
ps u $$       # info about the current shell ($$ = its PID)

Command	Effect
`kill <pid>`	Terminate (TERM)
`kill -STOP <pid>`	Freeze
`kill -CONT <pid>`	Resume
`kill -KILL <pid>`	Force kill (last resort)
`kill -l`	List signal numbers and names
`jobs`	Show suspended / background jobs
`cmd &`	Run command in background
`nohup cmd &`	Keep running after logout

5. File permissions

-rw-r--r-- 1 linuxize users 12288 Apr  8 20:51 filename.txt
|[-][-][-]-  [------] [---]
| |  |  | |     |       |
| |  |  | |     |       +------------> 7. Group
| |  |  | |     +--------------------> 6. Owner
| |  |  | +--------------------------> 5. Alternate access method
| |  |  +----------------------------> 4. Others permissions
| |  +-------------------------------> 3. Group permissions
| +----------------------------------> 2. Owner permissions
+------------------------------------> 1. File type

File types (first character of `ls -l`)

Symbol	Type	Example
`-`	Regular file	`/etc/passwd`
`d`	Directory	`/home`
`l`	Symbolic link	`/bin -> usr/bin`
`b`	Block device (data in blocks, e.g. disks)	`/dev/sda`
`c`	Character device (data as a stream, e.g. terminals)	`/dev/tty`, `/dev/null`
`p`	Named pipe (FIFO) — one process writes, another reads	`mkfifo mypipe`
`s`	Socket — local inter-process communication	`/run/docker.sock`

chmod — numeric values

Permission	Value
`r` read	4
`w` write	2
`x` execute	1
none	0

Example 754: owner rwx = 7, group r-x = 5, others r-- = 4.

umask — default permissions mask for newly created files

Links: hard vs symbolic

ln <target> <linkname>       # hard link
ln -s <target> <linkname>    # symbolic (soft) link

The difference is what the link points to. A hard link is another name for the same inode — the actual data on disk. A symbolic link is a separate small file that stores the path of the target.

	Hard link	Symbolic link
Points to	Inode (the data itself)	Path name of the target
`ls -l` shows	Regular file (`-`), link count > 1	`l`, e.g. `link -> target`
Target deleted	Data survives as long as one name remains	Link breaks (“dangling”)
Target moved/renamed	Still works	Breaks
Across filesystems	No (same filesystem only)	Yes
Link to a directory	No	Yes
Own permissions	Same as the file (it is the file)	Ignored — target’s permissions apply

Check with ls -li (the -i shows the inode number): hard links to the same file share one inode number, and the second column is the link count.

6. Archives and compression

gzip file           # -> file.gz
gunzip file.gz
zcat file.gz        # same as gunzip -dc (print to stdout)

tar cvf archive.tar file1 file2   # create
tar xvf archive.tar               # extract
tar tvf archive.tar               # list contents
tar ztvf file.tar.gz              # list a gzipped archive

Flag	Meaning
`c`	create
`x`	extract
`t`	test / list
`v`	verbose
`f`	next argument is the file
`p`	preserve permissions
`z`	gzip compression

7. Filesystem Hierarchy Standard

/
├── bin/      ready-to-run commands (essential binaries)
├── sbin/     system executables (for root)
├── boot/     kernel and boot loader files
├── dev/      device files
├── etc/      core system configuration (passwd, boot, network, ...)
├── home/     personal directories of regular users
├── lib/      shared and static libraries used by executables
├── media/    mount point for removable media (USB drives, ...)
├── opt/      additional third-party software
├── proc/     info about running processes and kernel parameters
├── run/      runtime data: PIDs, sockets, status records, logging
├── sys/      device and system interface (similar to /proc)
├── tmp/      small temporary files
├── var/      variable data: logs, caches, user tracking
└── usr/      user programs and data
    ├── include/  header files for the C compiler
    ├── local/    software installed by the administrator
    ├── man/      manual pages
    └── share/    architecture-independent files

Kernel location: /vmlinuz or /boot/vmlinuz.

8. Root and sudo

sudo <cmd>                            # run command as root
visudo                                # safely edit /etc/sudoers (who may use sudo)
journalctl SYSLOG_IDENTIFIER=sudo     # view sudo logs