What is an equivalent of the adduser command on Mac OS X?

There is a good script available at http://wiki.freegeek.org/index.php/Mac_OSX_adduser_script However it has several typos and sometimes is not flexible enough, so here is my modified version of it with some improvements:

#!/bin/bash
# =========================
# Add User OS X Interactive Command Line
# =========================

getHiddenUserUid()
{
    local __UIDS=$(dscl . -list /Users UniqueID | awk '{print $2}' | sort -ugr)

    #echo $__UIDS
    local __NewUID
    for __NewUID in $__UIDS
    do
        if [[ $__NewUID -lt 499 ]] ; then
            break;
        fi
    done

    echo $((__NewUID+1))
}

getInteractiveUserUid()
{
    # Find out the next available user ID
    local __MAXID=$(dscl . -list /Users UniqueID | awk '{print $2}' | sort -ug | tail -1)
    echo $((__MAXID+1))
}

if  [ $UID -ne 0 ] ; then echo "Please run $0 as root." && exit 1; fi

read -p "Enter your desired user name: " USERNAME

read -p "Enter a full name for this user: " FULLNAME

read -s -p "Enter a password for this user: " PASSWORD
echo
read -s -p "Validate a password: " PASSWORD_VALIDATE
echo

if [[ $PASSWORD != $PASSWORD_VALIDATE ]] ; then
    echo "Passwords do not match!"
    exit 1;
fi

# ====

# A list of (secondary) groups the user should belong to
# This makes the difference between admin and non-admin users.

read -p "Is this an administrative user? [y/n] (n): " GROUP_ADD
GROUP_ADD=${GROUP_ADD:-n}

if [ "$GROUP_ADD" = n ] ; then
    SECONDARY_GROUPS="staff"  # for a non-admin user
elif [ "$GROUP_ADD" = y ] ; then
    SECONDARY_GROUPS="admin _lpadmin _appserveradm _appserverusr" # for an admin user
else
    echo "You did not make a valid selection!"
    exit 1;
fi

# ====

# Create a UID that is not currently in use

read -p "Should this user have interactive access?  [y/n] (y): " IS_INTERACTIVE
IS_INTERACTIVE=${IS_INTERACTIVE:-y}

if [ "$IS_INTERACTIVE" = y ] ; then
    USERID=$(getInteractiveUserUid)
elif [ "$IS_INTERACTIVE" = n ] ; then
    USERID=$(getHiddenUserUid)
else
    echo "You did not make a valid selection!"
    exit 1;
fi

echo "Going to create user as:"
echo "User name: " $USERNAME
echo "Full name: " $FULLNAME
echo "Secondary groups: " $SECONDARY_GROUPS
echo "UniqueID: " $USERID

read -p "Is this information correct?  [y/n] (y): " IS_INFO_CORRECT
IS_INFO_CORRECT=${IS_INFO_CORRECT:-y}

if [ "$IS_INFO_CORRECT" = y ] ; then
    echo "Configuring Open Directory..."
elif [ "$IS_INFO_CORRECT" = n ] ; then
    echo "User creation cancelled!"
    exit 1;
else
    echo "You did not make a valid selection!"
    exit 1;
fi


# Create the user account by running dscl (normally you would have to do each of these commands one
# by one in an obnoxious and time consuming way.

dscl . -create /Users/$USERNAME
dscl . -create /Users/$USERNAME UserShell /bin/bash
dscl . -create /Users/$USERNAME RealName "$FULLNAME"
dscl . -create /Users/$USERNAME UniqueID "$USERID"
dscl . -create /Users/$USERNAME PrimaryGroupID 20
dscl . -create /Users/$USERNAME NFSHomeDirectory /Users/$USERNAME
dscl . -passwd /Users/$USERNAME $PASSWORD


# Add user to any specified groups
echo "Adding user to specified groups..."

for GROUP in $SECONDARY_GROUPS ; do
    dseditgroup -o edit -t user -a $USERNAME $GROUP
done

# Create the home directory
echo "Creating home directory..."
createhomedir -c 2>&1 | grep -v "shell-init"

echo "Created user #$USERID: $USERNAME ($FULLNAME)"

I think you're looking for the dscl command. Niutil sounds like NetInfo which is deprecated in OS X in favor of Open Directory aka LDAP.

Dscl, or Directory Services Command Line, can be used to edit Open Directory for both a local or remote user data store. It can be done with sudo commands but it's easier to use as root.

Here's a short and highly inadequate tutorial: In the terminal, sudo -s to switch your user to root. To create a functional user account named dscl2, you need to do the following:

dscl . -create /Users/dscl2

dscl . -create /Users/dscl2 UserShell /bin/bash

dscl . -create /Users/dscl2 RealName "DSCL 2"

dscl . -create /Users/dscl2 UniqueID 8005

dscl . -create /Users/dscl2 PrimaryGroupID 20

dscl . -create /Users/dscl2 NFSHomeDirectory /Users/dscl2

dscl . -passwd /Users/dscl2 password

The UUID is typically around 501 or larger. 501 is the default UID for the first account created. UIDs less than 500 don't show up in the Accounts pane by default. Choose whatever number you want, but make sure it is unique on the local system. Don't overwrite an existing UID or you will have big problems.

Dscl has an interactive mode as well that works a little differently. Enter just "dscl" at the prompt to enter interactive mode.

If you're in Interactive mode, type ls to list which directories are available. You should see BSD, LDAP and Local. You navigate through the directories with cd. See your friend the man page for more info.