When the bash command is executed a new shell is opened. The $SHLVL environment variable holds the number of shell levels the current shell is running on top of.
In a new terminal window, executing the following command will produce different results based on the Linux distribution in use.
Using Fedora 25, the output is "3". This indicates, that when opening a new shell, an initial bash command executes and performs a task. The initial bash command executes a child process (another bash command) which, in turn, executes a final bash command to open the new shell. When the new shell opens, it is running as a child process of 2 other shell processes, hence the output of "3".
In the following example (given the user is running Fedora 25), the output of $SHLVL in a new shell will be set to "3". As each bash command is executed, $SHLVL increments by one.
~> $ echo $SHLVL 3 ~> $ bash ~> $ echo $SHLVL 4 ~> $ bash ~> $ echo $SHLVL 5
One can see that executing the 'bash' command (or executing a bash script) opens a new shell. In comparison, sourcing a script runs the code in the current shell.
#!/usr/bin/env bash echo "Hello from test1.sh. My shell level is $SHLVL" source "test2.sh"
#!/usr/bin/env bash echo "Hello from test2.sh. My shell level is $SHLVL"
#!/usr/bin/env bash echo "Hello from run.sh. My shell level is $SHLVL" ./test1.sh
chmod +x test1.sh && chmod +x run.sh ./run.sh
Hello from run.sh. My shell level is 4 Hello from test1.sh. My shell level is 5 Hello from test2.sh. My shell level is 5