The shebang must be the first line, because it is interpreted by the kernel, which looks at the two bytes at the start of an executable file. If these are #! the rest of the line is interpreted as the executable to run and with the script file available to that program. (Details vary slightly, but that is the picture).
Since the kernel will only look at the first two characters and has no notion of further lines, you must place the hash bang in line 1.
If a shell script has a zsh shebang, when you run it in bash shell with dot slash:
zzh@ZZHPC:~/aaa$ ./test_zsh_shebang.sh sleep 500s ^C zzh@ZZHPC:~/aaa$ cat test_zsh_shebang.sh #!/bin/zsh echo 'sleep 500s' sleep 500 zzh@ZZHPC:~/aaa$ echo $SHELL /bin/bash zzh@ZZHPC:~/aaa$ ./test_zsh_shebang.sh sleep 500s
You'll see it is run by zsh:
zzh@ZZHPC:~$ psvgrep test_zsh_shebang zzh 6957 5189 0 10:44 pts/0 00:00:00 /bin/zsh ./test_zsh_shebang.sh
If a shell script has a bash shebang, when you run it in zsh shell with dot slash:
zzh@ZZHPC:~/aaa$ zsh 2024-05-17 11:01AM [zzh@(ZZHPC):/home/zzh/aaa]$>echo $SHELL /bin/bash 2024-05-17 11:01AM [zzh@(ZZHPC):/home/zzh/aaa]$>cat test_bash_shebang.sh #!/bin/bash echo 'sleep 500s' sleep 500 2024-05-17 11:02AM [zzh@(ZZHPC):/home/zzh/aaa]$>./test_bash_shebang.sh sleep 500s
You'll see it is run by bash:
zzh@ZZHPC:~$ psvgrep test_bash_shebang zzh 6983 6972 0 10:47 pts/0 00:00:00 /bin/bash ./test_bash_shebang.sh
If a shell script doesn't have a shebang line, when you run it wit dot slash:
In bash shell (Ubuntu default shell):
zzh@ZZHPC:~/aaa$ cat test_no_shebang.sh echo 'sleep 500s' sleep 500 zzh@ZZHPC:~/aaa$ ./test_no_shebang.sh sleep 500s
zzh@ZZHPC:~$ alias psvgrep alias psvgrep='ps -ef | grep -v grep | grep' zzh@ZZHPC:~$ psvgrep test_no_shebang zzh@ZZHPC:~$ psvgrep sleep zzh 6999 6998 0 10:51 pts/0 00:00:00 sleep 500 zzh@ZZHPC:~$ psvgrep 6998 zzh 6998 5189 0 10:51 pts/0 00:00:00 /bin/bash zzh 6999 6998 0 10:51 pts/0 00:00:00 sleep 500
it is run by bash, and you CANNOT FIND THE PROCESS BY GREPPING THE SCRIPT NAME in the output of 'ps -ef'.
In zsh shell:
zzh@ZZHPC:~/aaa$ zsh 2024-05-17 11:05AM [zzh@(ZZHPC):/home/zzh/aaa]$>echo $SHELL /bin/bash 2024-05-17 11:05AM [zzh@(ZZHPC):/home/zzh/aaa]$>./test_no_shebang.sh sleep 500s
zzh@ZZHPC:~$ psvgrep test_no_shebang zzh 7172 7169 0 11:05 pts/0 00:00:00 sh ./test_no_shebang.sh zzh@ZZHPC:~$ which sh /usr/bin/sh zzh@ZZHPC:~$ lh /usr/bin/sh lrwxrwxrwx 1 root root 4 Jun 12 2023 /usr/bin/sh -> dash zzh@ZZHPC:~$ lh /usr/bin/dash -rwxr-xr-x 1 root root 123K Mar 23 2022 /usr/bin/dash
it is run by sh, which is a soft link pointing to /usr/bin/dash, and you can find the process by grepping the script name in the output of 'ps -ef'.