A list of useful commands

User management

# create a user
USERNAME=foo
useradd $USERNAME

# reset password
USERNAME=foo
passwd $USERNAME

# add an existing user to a group
USERNAME=foo
GROUP=www
usermod -a -G $GROUP $USERNAME
cat /user/group

# remove user
USERNAME=foo
userdel $USERNAME

# add a user to a group
USERNAME=foo
GROUP=www-data
usermod -a -G $GROUP $USERNAME

Archive

# Create an archive
OUTPUT=archive.tar.gz
INPUT=folder
tar -zcvf $OUTPUT $INPUT

# Extract archive
OUTPUT=folder
INPUT=archive.tar.gz
tar -zxvf $INPUT -C $OUTPUT

Utilities

# disk usage
df -h
# find out who is taking too much place
du -hsx /* | sort -rh

network

IP_TO_CHECK=0.0.0.0
nmap -PN $IP_TO_CHECK

GIT

# change commit timestamp
LC_ALL=C GIT_COMMITTER_DATE="$(date)" git commit --amend --no-edit --date "$(date)"

I will continue to add on here as I deem fit.