Not able to set password in ArchLinux

Easiest way to do this from the command line is to use the passwd command with root privileges.

passwd username

From man 1 passwd

NAME
       passwd - update user's authentication token
SYNOPSIS
       passwd  [-k]  [-l]  [-u [-f]] [-d] [-n mindays] [-x maxdays]
       [-w warndays] [-i inactivedays] [-S] [--stdin] [username]
DESCRIPTION
       The passwd utility is used to update user's authentication token(s).

After you set the user password, you can force the user to change it on next login using the chage command (also with root privileges) which expires the password.

chage -d 0 username

When the user successfully authenticates with the password you set, the user will automatically be prompted to change it. After a successful password change, the user will be disconnected, forcing re-authentication with the new password.

See man 1 chage for more information on password expiry.

You can use chpasswd

The chpasswd command administers users' passwords. The root user can supply or change users' passwords specified through standard input. Each line of input must be of the following format.

username:password Only root users can set passwords with this command.

Example 1:

echo username:password | chpasswd 

Example 2:

Also for security you can pass encrypted password to chpasswd

# Create Password in Encrpyted Form Using below command
# perl -e'print crypt("YourPassword", "salt")' ; echo -e

echo username:cryptedPass | chpasswd -e