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]

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

apt install fortune/fortune-mod/fortunes-zh

man 6 fortune

fortune - print a random, hopefully interesting, adage

fortune | cowsay | lolcat

man pwd

pwd - print name of current/working directory

man cat

cat - concatenate files and print on the standard output

cat -n FILE

cat -b FILE

man ls

ls - list directory contents

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

ls -ld */

ls -ld .*/ */

ls -ld /usr

ls -lF /

ls ~ | wc -l

ls ~ | od -c

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

ls ~ | cat -A

file$
foo$
foobar$

man file

file — determine file type

file /dev/tty

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

file /dev/vda

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

file /bin

/bin: symbolic link to usr/bin

file /bin/

/bin/: directory

man less

less - opposite of more

man usermod

usermod - modify a user account

usermod -s /sbin/nologin kuga

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

usermod -aG sudo kuga

man ln

ln - make links between files

man df

df - report file system disk space usage

df -h

df -h -t ext4

df -h -t apfs

man du

du - estimate file space usage

du -sh ~/Downloads

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

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

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