A shell script that can run under different shells

I came up with three methods.

Exit early

Use exit early instead of the elif syntax. Basically shell is an interpreter. As soon as the parsing of the first if statement is finished, it will be executed. In this case, if shell is bash, exit before the parsing of the next if statement begins.

if [ -n "$BASH_VERSION" ]; then
    for f in *; do echo __BASH__ $f; done
    exit
fi

if [ -n "$ZSH_VERSION" ]; then
    for f (*) echo __ZSH__ $f
fi

Eval string

Use eval.

if [ -n "$BASH_VERSION" ]; then
    for f in *; do echo __BASH__ "$f"; done
elif [ -n "$ZSH_VERSION" ]; then
    eval 'for f (*) echo __ZSH__ "$f"'
fi

Source here-document

This is the same as Canina's answer. However, you can write the code in the same file by using here-document.

if [ -n "$BASH_VERSION" ]; then
    for f in *; do echo __BASH__ "$f"; done
elif [ -n "$ZSH_VERSION" ]; then
    source /dev/stdin << '__ZSH_SRC__'
        for f (*) echo __ZSH__ "$f"
__ZSH_SRC__
fi

posted about 2 years ago

CC0

mjy‭

11 reputation 0 1 1 0

I'm not familiar with zsh, but it seems to me that your problem here is that the syntax for for loops is different in bash and zsh, which throws bash off as it tries to interpret your script, finds a keyword it knows but the rest of the statement doesn't match what it expects.

The solution would seem to me to be to put the shell-specific parts in separate files, and only source the file matching the current execution environment into the script. So you might have a main script like:

#!/usr/bin/env bash

shell=$(ps -p $$ -oargs= | awk '{print $1}')

if [ $shell = "bash" ]; then
    source ./bash-source.sh
elif [ $shell = "zsh" ]; then
    source ./zsh-source.sh
fi

as well as bash-source.sh

for f in ~/.functions.d/*.sh; do source $f; done

and zsh-source.sh

for f (~/.functions.d/**/*.sh) source $f

This way, each shell only sees the for statement with the correct syntax for it, and therefore doesn't get tripped up by trying to parse the other. This relies on the fact that at least bash doesn't actually parse a file until it's loaded, and loading happens only when the source statement is actually executed.

If zsh deals gracefully with this situation, you might not need to split the bash source loop into its own file; but clearly bash doesn't like to see the zsh for (as evidenced by the fact that you're getting an error relating to it), and thus bash needs a little hand-holding.

Also note that I had to add | awk '{print $1}' to the $shell assignment. Without it, when within a bash shell script, $shell was assigned the whole execution command line; in other words, bash ./filename.sh even when executing the script only as ./filename.sh. There's almost certainly a more efficient way to do that; for the moment, I simply needed something that got me past that assignment and would likely give the intended value. This does however suggest some system-dependence in the ps output that you may need to deal with, depending on just how portable you want this script to be.

posted over 2 years ago

CC BY-SA 4.0

Canina‭

1116 reputation 4 24 139 35