In some cases, I got to get the user/group's name from its id. Here's how-to.

group-name.sh

# provide some group's id
$gid=1000

# POSIX way
cat /etc/group | grep ":x:$gid:" | cut -d: -f1

# non-POSIX way which also handles LDAP
getent group "$gid" | cut -d: -f1

# Note that we can't use `id` command for this purpose.
# It always handles an argument as an user id, not group id.

user-name.sh

# provide some user's id
$uid=1000

# POSIX way 1
cat /etc/passwd | grep ":x:$uid:" | cut -d: -f1

# POSIX way 2, but sometimes doesn't work
id -un "$uid"

# non-POSIX way which also handles LDAP
getent passwd "$uid" | cut -d: -f1

# See also https://unix.stackexchange.com/questions/36580/how-can-i-look-up-a-username-by-id-in-linux