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

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.
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.

Answer 2

In simple words, sh file1 executing sh command/executable with file1 as a parameter. In this case file1 doesn't require execute privilege as sh executable read and intercept the commands in the file.
./file1 its nothing but running/executing an executable file file1, hence it requires executable privileges. In this case it executes on the shell mentioned in the shebang #!/bin/sh if its not mentioned then its on the current shell.
Hoping the above statements are not chaos :)

Answer 3

With sh , we can run a script that doesn’t have execute permission set on it, we run it as argument for sh, but ./ needs the permission as it is supposed to be an executable.
In both cases, new shell will be created to run the script. See the below example:
root#ub18:~/shell# vi test1.sh
#!/bin/bash
my_var=hello
echo $my_var
#Shows the current shell processid
echo $$
root#ub18:~/shell# echo $$
1896
root#ub18:~/shell# sh test1.sh
hello
2093
root#ub18:~/shell# ./test1.sh
-su: ./test1.sh: Permission denied
root#ub18:~/shell# chmod +x ./test1.sh
root#ub18:~/shell# ./test1.sh
hello
2102
root#ub18:~/shell# ./test1.sh
hello
2103
root#ub18:~/shell# ./test1.sh
hello
2104
root#ub18:~/shell# sh test1.sh
hello
2106

Answer 4

when your file is not executable you can't run using ./file_name.sh normally you can run by sh file_name.sh if you change your file to executable chmod a+x file_name.sh you can run your file by ./file_name.sh