Common Commands 1

MAN

This command can view different sections of the manual pages.

man man

man - an interface to the system reference manuals

The table below shows the section numbers of the manual followed by the types of pages they contain.

1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions, e.g. /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]

Section

man 1 ls/cp/mv
man 2 write/open/close
man 3 printf/malloc/free
man 4 tty/null/random
man 5 passwd/fstab
man 7 man
man 8 ifconfig/iptables/mount
man 9 fork # not work

Games

fortune can randomly display quotes and jokes.

apt install fortune/fortune-mod/fortunes-zh

man 6 fortune

fortune - print a random, hopefully interesting, adage

Can be combined with cowsay and lolcat to play.

fortune | cowsay | lolcat

PWD

Print Working Directory.

man pwd

pwd - print name of current/working directory

CAT

man cat

cat - concatenate files and print on the standard output

Line No. With Blank Lines

cat -n FILE

Line No. Without Blank

cat -b FILE

LS

man ls

ls - list directory contents

File Details

ls -lh

drwx------ 9 root root 4.0K   Aug  7 22:38 root
crw-rw-rw- 1 root tty  5,   0 Aug  7 22:33 /dev/tty
brw-rw---- 1 root disk 252, 0 Aug  7 21:30 /dev/vda

File type: d directory, - file, l link, c character device, b block device.
File permissions: 700, Owner: 7, Group: 0, Other: 0
Hard link count: 9
File owner: root
File group: root
File size: 4.0K
Modification time: Aug 7 22:38
File name: root
Major device number: 5/252, identifies the device driver
Minor device number: 0, identifies a specific device instance within the same class of devices

Only Dirs

ls -ld */

Including hidden files

ls -ld .*/ */

Specify Dir

ls -ld /usr

Type Symbols

ls -lF /

/: directory
*: executable file
@: symbolic link
|: named pipe
=: socket

Hidden Characters

ls ~ | wc -l

There are hidden characters \n in the output of the ls command.

ls ~ | od -c

0000000   f   i   l   e  \n   f   o   o  \n   f   o   o   b   a   r  \n
0000020

You can also use cat -A.

ls ~ | cat -A

file$
foo$
foobar$

FILE

Show file type

man file

file — determine file type

Character/Block Device

file /dev/tty

/dev/tty: character special (5/0)

file /dev/vda

/dev/vda: block special (252/0)

Link File

Note the trailing slash.

file /bin

/bin: symbolic link to usr/bin

file /bin/

/bin/: directory

LESS

The more command can only load by pressing Enter, the less command is more powerful because less is more.

man less

less - opposite of more

USERMOD

man usermod

usermod - modify a user account

Disable Account Login

usermod -s /sbin/nologin kuga

This command will change bin/sh to /sbin/nologin in /etc/passwd

kuga:x:1000:1000::/home/kuga:/sbin/nologin

Grant User Group

The following command will grant the kuga user the sudo user group.

usermod -aG sudo kuga

LN

man ln

ln - make links between files

Soft Link

go rocks

DF

man df

df - report file system disk space usage

Readable Format

df -h

Specify File System Type

df -h -t ext4

df -h -t apfs

ext4 is the file system type of Linux, apfs is the file system type of macOS.

DU

Disk Usage

man du

du - estimate file space usage

Total Size of Dir

du -sh ~/Downloads

Sort Files - Including Subdirs

du -ah ~/Downloads/* | sort -rh | head -n 5

du -ah ~/Downloads --max-depth=1 | sort -rh | head -n 10

Sort Files - Excluding Subdirs

du -sh ~/Downloads/* | sort -rh | head -n 5