Portable sed -i across MacOS and Linux

The -i flag to ask sed to edit a file in place works differently on Linux and MacOS. If you want to create a backup of your file before you edit it, say with the extension .bak, then on Linux you would run

    sed -i.bak myfile

but for the version of sed that ships with MacOS you would write

    sed -i '.bak' myfile

Note that this changes how sed interprets its arguments, whether you want a backup file or not. You must specify a backup file extension, but could specify the extension as '', which effectively means don’t make a backup.

The difference between how the two versions of sed handle their arguments is a minor nuisance when working interactively at the command line, but a script calling sed -i will not work the same on Mac and Linux.

I put the line

   alias sed=gsed

in my .zshrc file so that when I type sed the shell will actually run the Gnu version of sed, which handles -i as I expect.

But this does not work in bash scripts. I tried putting the alias in my .bashrc and .bash_profile files, but that doesn’t work. In scripts bash ignores aliases, no matter what config file you put them in. Here’s the relevant line from the bash man page:

Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt.

So the solution is to put the magic incantation

   shopt -s expand_aliases

at the top of your script.

Here’s how I wrote a script using sed -i works the same way on MacOS and Linux.

    #!/usr/bin/env bash
    shopt -s expand_aliases

    if [[ "$OSTYPE" == "darwin"* ]]; then
        alias sed=gsed
    fi

    sed -i ...

This may seem a little heavy-handed, changing the program that I’m using just to fix a problem with the arguments. I could, for example, have the script insert '' after -i when running on MacOS.

But I’m not changing the program I use. Quite the opposite. The code above saves me from having to use a different program. It insures that I’m running the Gnu version of sed on both platforms. There are other differences between the two versions of sed. I don’t know what they are off hand, but they could lead to frustrating bugs in the future, and the code above fixes these bugs before they occur.