Path for bash in Android termux Regular paths for bash do not work in termux app. 
I tried: 
/usr/bin/bash 
and 
/bin/bash 
Also 'whereis' command gives the following output: 
$ whereis bash bash: 
/data/data/com.termux/files/usr/bin/bash 
But this path also is not correct. So, I have to run every bash script with the word 'bash' before it. And cannot run bash scripts without it.
In September 2017 the maintainer of Termux released a package termux-exec, which wraps up execve(2) so that files that has a shebang line like #!/bin/sh or #!/usr/bin/env will run correctly in Termux. Just run

pkg install termux-exec


and restart Termux (or open a new session). You'll now be able to run #!/bin/sh scripts.

The previous solution was as following:


  Termux provides a handy utility

termux-fix-shebang

  
  Whose description reads:
  
  
    Rewrite shebangs in specified files for running under Termux,
    which is done by rewriting #!*/bin/binary to #!$PREFIX/bin/binary.
  
  
  Just apply it to the scripts you wish to run. It does what its name suggests: fix the shebang line #!/xxx of your script files. It can also fix other scripts like Perl or Python.


Also note that by default your working directory is not in $PATH, so you cannot directly type myscript.sh, but instead

./myscript.sh
^~


... or explicitly specify an interpreter shell (in which case you don't need the directory prefix):

bash myscript.sh
^~~~