shell - unix command line execute with . (dot) vs. without

Question

At a unix command line, what's the difference between executing a program by simply typing it's name, vs. executing a program by typing a . (dot) followed by the program name? e.g.:

runme

vs.

. runme

Answer 1

. name sources the file called name into the current shell. So if a file contains this

A=hello

Then if you sources that, afterwards you can refer to a variable called A which will contain hello. But if you execute the file (given proper execution rights and #!/interpreterline), then such things won't work, since the variable and other things that script sets will only affects its subshell it is run in.

Sourcing a binary file will not make any sense: Shell wouldn't know how to interpret the binary stuff (remember it inserts the things appearing in that file into the current shell - much like the good old #include <file> mechanism in C). Example:

head -c 10 /dev/urandom > foo.sh; . foo.sh # don't do this at home!
bash: ??D$?/?: file or directory not found

Executing a binary file, however, does make a lot of sense, of course. So normally you want to just name the file you want to execute, and in special cases, like the A=hello case above, you want to source a file.