Linux: Basic Command

2026-09-02

1. Shell basics

Manual page sections

The manual is split into numbered sections, and the same name can exist in several of them — passwd(1) is the command, passwd(5) is the file format of /etc/passwd. The number in parentheses after a name (ls(1), open(2)) tells you which section it comes from.

man passwd          # first match — section 1 (the command)
man 5 passwd        # the /etc/passwd file format
man -a passwd       # show every section that has a page, one after another
man -f passwd       # list sections where the name exists (same as whatis)
man -k <keyword>    # search descriptions (same as apropos)

The default search order is roughly 1, 8, 2, 3, 5, … (see man man), so man with no number usually gives you the command first.

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

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)

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)

chmod — numeric values

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.

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.

Sources: CBT Nuggets, GeeksforGeeks

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

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