Difference between "./" and "sh" in UNIX

Question

Sometimes i see that few scripts are executed through "sh" command and sometimes through "./" command.I am not able to understand the exact difference between them.Please help me out .

Answer 1

sh file executes a shell-script file in a new shell process.

. file executes a shell-script file in the current shell process.

./file will execute the file in the current directory. The file can be a binary executable, or it can start with a hashbang line (the first line of the file in form of #!...., for example #!/usr/bin/ruby in a file would signify the script needs to be executed as a Ruby file). The file needs to have the executable flag set.

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For example, if you have the script test.sh:

#!/bin/sh  TEST=present 

and you execute it with sh test.sh, you'd launch a new sh (or rather bash, most likely, as one is softlinked to the other in modern systems), then define a new variable inside it, then exit. A subsequent echo $TEST prints an empty line - the variable is not set in the outer shell.

If you launch it using . test.sh, you'd execute the script using the current shell. The result of echo $TEST would print present.

If you launch it using ./test.sh, the first line #!/bin/sh would be detected, then it would be exactly as if you wrote /bin/sh ./test.sh, which in this case boils down to the first scenario. But if the hashbang line was, for example, #!/usr/bin/perl -w, the file would have been executed with /usr/bin/perl -w ./test.sh.