removed /etc/hosts on a mac and now can't sudo
The file /etc/hosts is in no way related to the ability to run sudo. You probably hosed your sudoers file in a previous step.

To solve your problem boot to Recovery Mode and open Terminal in the menubar > Utilities.


Run df to get the name of your main system volume.
Change your working directory to /Volumes//etc:

cd /Volumes/Macintosh\ HD/etc #"Macintosh HD" is just an example name here

Copy the spare files hosts~orig and sudoers~orig (or with similar names which may exist as "backup" in /etc by default - check this with ls!):

cp hosts~orig hosts
cp -i sudoers~orig sudoers #enter yes here


If they don't exist create the hosts file and and check your sudoers file:

/Volumes/&lt;name_of_main_volume&gt;/usr/bin/nano /Volumes/&lt;name_of_main_volume&gt;/etc/hosts


The content of the default hosts file is.

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1   localhost
255.255.255.255 broadcasthost
::1             localhost 


The POSIX permissions are root:wheel 644. Modify this with chown and chmod if necessary.

/Volumes//usr/bin/nano /Volumes//etc/sudoers

A minimal (to type less!) working sudoers file looks like this:

Defaults    env_reset
Defaults    env_keep += "BLOCKSIZE"
Defaults    env_keep += "COLORFGBG COLORTERM"
Defaults    env_keep += "__CF_USER_TEXT_ENCODING"
Defaults    env_keep += "CHARSET LANG LANGUAGE LC_ALL LC_COLLATE LC_CTYPE"
Defaults    env_keep += "LC_MESSAGES LC_MONETARY LC_NUMERIC LC_TIME"
Defaults    env_keep += "LINES COLUMNS"
Defaults    env_keep += "LSCOLORS"
Defaults    env_keep += "SSH_AUTH_SOCK"
Defaults    env_keep += "TZ"
Defaults    env_keep += "DISPLAY XAUTHORIZATION XAUTHORITY"
Defaults    env_keep += "EDITOR VISUAL"
Defaults    env_keep += "HOME MAIL"

Defaults    lecture_file = "/etc/sudo_lecture"

root        ALL = (ALL) ALL
%admin      ALL = (ALL) ALL


The POSIX permissions are root:wheel 440. Modify this with chown and chmod if necessary.
Reboot your Mac by entering:

reboot



This can also be done in single-user-mode after mounting the root volume rw.



After regaining the ability to run sudo, either extract it from the macOS installer app with Pacifist or from a backup or modify it yourself with visuso. The default content is:

#
# Sample /etc/sudoers file.
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the sudoers man page for the details on how to write a sudoers file.

##
# Override built-in defaults
##
Defaults    env_reset
Defaults    env_keep += "BLOCKSIZE"
Defaults    env_keep += "COLORFGBG COLORTERM"
Defaults    env_keep += "__CF_USER_TEXT_ENCODING"
Defaults    env_keep += "CHARSET LANG LANGUAGE LC_ALL LC_COLLATE LC_CTYPE"
Defaults    env_keep += "LC_MESSAGES LC_MONETARY LC_NUMERIC LC_TIME"
Defaults    env_keep += "LINES COLUMNS"
Defaults    env_keep += "LSCOLORS"
Defaults    env_keep += "SSH_AUTH_SOCK"
Defaults    env_keep += "TZ"
Defaults    env_keep += "DISPLAY XAUTHORIZATION XAUTHORITY"
Defaults    env_keep += "EDITOR VISUAL"
Defaults    env_keep += "HOME MAIL"

Defaults    lecture_file = "/etc/sudo_lecture"

##
# User alias specification
##
# User_Alias    FULLTIMERS = millert, mikef, dowdy

##
# Runas alias specification
##
# Runas_Alias   OP = root, operator

##
# Host alias specification
##
# Host_Alias    CUNETS = 128.138.0.0/255.255.0.0
# Host_Alias    CSNETS = 128.138.243.0, 128.138.204.0/24, 128.138.242.0
# Host_Alias    SERVERS = master, mail, www, ns
# Host_Alias    CDROM = orion, perseus, hercules

##
# Cmnd alias specification
##
# Cmnd_Alias    PAGERS = /usr/bin/more, /usr/bin/pg, /usr/bin/less

##
# User specification
##

# root and users in group wheel can run anything on any machine as any user
root        ALL = (ALL) ALL
%admin      ALL = (ALL) ALL

## Read drop-in files from /private/etc/sudoers.d
## (the '#' here does not indicate a comment)
#includedir /private/etc/sudoers.d