Shell

sh (or the Shell Command Language) is a programming language described by the POSIX standard. It has many implementations (ksh88, dash, …).

bash can also be considered an implementation of sh.

Command

#!/bin/sh will use sh (and whatever that happens to point to). For a long time, it is a symlink or hardlink to /bin/bash by default in many OS.

#!/bin/bash will use /bin/bash if it’s available (and fail with an error message if it’s not).

Of course, you can also specify another implementation, e.g. #!/bin/dash

$ source a.sh
$ . a.sh # There is space between . and a.sh

Execute in current shell and keep environment change from a.sh.

$ sh a.sh
$ bash a.sh

Execute in a subshell and environment does not change in parent shell.

./a.sh

Need permission to execute in a subshell and environment does not change in parent shell.

./a.sh fork
./a.sh source
./a.sh exec

fork creates a child process to execute a.sh and will not change the current environment after execution.

source executes a.sh in current process and will change the environment according to a.sh.

exec is similar to source but will terminate current process.

By default, fork is applied.

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