Getting "sqlcmd sqlcmd: command not found" in Linux?
There are two methods of resolving this.

User-specific method advocated in the manual: add the directory to the environmental variable PATH.
System-wide method: symlink the executable to a directory already in the path.

User-specific Method
You can use the tutorial's recommended method. The tutorial states the following:

Optional: Add /opt/mssql-tools/bin/ to your PATH environment variable in a bash shell.
To make sqlcmd/bcp accessible from the bash shell for login sessions, modify your PATH in the ~/.bash_profile file with the following command:
echo 'export PATH=&quot;$PATH:/opt/mssql-tools/bin&quot;' &gt;&gt; ~/.bash_profile

To make sqlcmd/bcp accessible from the bash shell for interactive/non-login sessions, modify the PATH in the ~/.bashrc file with the following command:
echo 'export PATH=&quot;$PATH:/opt/mssql-tools/bin&quot;' &gt;&gt; ~/.bashrc
source ~/.bashrc


This works, but it only affects that user's sessions, and only if they use bash.
System-wide Method
Link the MS SQL tools into a location accessible to all users:
sudo ln -s /opt/mssql-tools/bin/* /usr/local/bin/

This could be in their postinst script, which would make sense for installing an optional package.
Both methods are compliant with the File system Hierarchy Standard, which states:

Programs to be invoked by users must be located in the directory /opt/&lt;package&gt;/bin