Shell Environment – Create Separate Environment with Different /bin/sh

bashshebangshell-script

I have a bunch of shell scripts which incorrectly assume /bin/sh to be equivalent to /bin/bash. E.g., they have the #!/bin/sh shebang, but use the source command instead of . (dot).

I run Ubuntu 16, where /bin/sh links to dash, and thus bash-isms are not supported.

I need to run the scripts periodically. Also, from time to time I will need to update them from the original author, who is not into fixing this particular bug.
I would like to avoid fixing all these files (there are a bunch of them, they are not mine, and I'll loose all the changes after update). Also, I would like to avoid making global changes to system, since it might potentially break other scripts.

Is there a way to somehow create a (temporary or not) environment with /bin/sh pointing to bash, to be used for these scripts, while not touching the global system /bin/sh?

Best Answer

You can easily fix them, don't break your system!

find . -name '*.sh' -type f -exec sed -i '1s|^#! */bin/sh|#!/bin/bash|' {} +

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Shell – how to share environment variables between shells like globals related to a master pid

You can write the environment to a file with export -p and read it back periodically in the other instances with . (the include command). But as you note, if there are concurrent modifications, the result won't be pretty.

If you want modifications to propagate instantly, you'll have to use a wrapper around the export builtin that also propagates the changes to the other shells. On Linux, you can use the flock utility.

global_export () {
  {
    flock 0
    . /path/to/env
    [ $# -eq 0 ] || export "$@"
    export -p >/path/to/env
  } </path/to/env
}

Note that you'll need to call global_export whenever you assign a new value to an exported variable.

This is rather clumsy. It's likely that there's a better way to solve your problem. The most obvious way is to have whatever command is using those environment variable read a configuration files instead.

Bash – Run a script on multiple shells

It would probably be easier to have two separate versions of the script. Since it is short it may not be worth adding extra code to handle the differences between the two formats as jw013 suggested. On the other hand if you had a larger script it would probably be easier to have one and make the script execute different commands depending on where it is running from.

Best Answer

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Shell – how to share environment variables between shells like globals related to a master pid

Bash – Run a script on multiple shells

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