09 August 2017

Shebang

This might seem obvious, but clearly it isn't. Rob Miller and I were pairing on a Friday afternoon and ran across a script in our SALT configuration. In the script Jinja was used to setup various variables before the script body. The Jinja code proceeded the shebang line and so the resulting script placed the shebang several lines into the file, with blank lines proceeding it. As a consequence, the shebang line was not executed. We dutifully fixed this in about six locations in our SALT config. 

We thought we should share our learning/remembering with the world. So we whipped up this simple example on a laptop. 

The following script (x.sh) changes the shell to zsh via the shebang and then prints the zsh version via echo. We then run the script and see that the version is 5.2. After that we create a new script (y.sh) with five blank lines followed by the content of x.sh. When we run that script we get no zsh version. That is because the shebang was ignored, its not the very first line of the file. 

Our point is, the magic number/shebang must be the first two bytes of the file or the shell will ignore it.