Bash Quoting – Difference Between Single Quoted $’string’ and Double Quoted $”string” in Bash

./test.sh runs test.sh as a separate program. It may happen to be a bash script, if the file test.sh starts with #!/bin/bash. But it could be something else altogether.

. ./test.sh executes the code of the file test.sh inside the running instance of bash. It works as if the content file test.sh had been included textually instead of the . ./test.sh line. (Almost: there are a few details that differ, such as the value of $BASH_LINENO, and the behavior of the return builtin.)

source ./test.sh is identical to . ./test.sh in bash (in other shells, source may be slightly different or not exist altogether; . for inclusion is in the POSIX standard).

The most commonly visible difference between running a separate script with ./test.sh and including a script with the . builtin is that if the test.sh script sets some environment variables, with a separate process, only the environment of the child process is set, whereas with script inclusion, the environment of the sole shell process is set. If you add a line foo=bar in test.sh and echo $foo at the end of the calling script, you'll see the difference:

$ cat test.sh
#!/bin/sh
foo=bar
$ ./test.sh
$ echo $foo

$ . ./test.sh
$ echo $foo
bar

If the delimiters may appear several times per lines, you could use perl instead like:

between() {
  perl -Tlne 'BEGIN{$b=shift;$e=shift}
             print for /\Q$b\E(.*?)\Q$e\E/g' "$@"
}

And then for example:

$ echo "[b]test[e] foo [b]bar[e]" | between '[b]' '[e]'
test
bar

You can also use it as:

between BEG END file1 file2...