Can the list if the variable used in a for loop be made up of variables themselves?

Question

I'm trying to create a script where pretty much things got sorted out earlier in it and I'm left with the booleans I need stored in variables. I just need to run them through one last loop and I'm done

This last part I'm talking about is a less readable version of something similar to this:

                         [↑continues on top↑]
var5="$(some long operation that has different results on different systems   )"
var6="$(bc of escaping or slighly different variations on the basic set of    )"
var7="$(UNIXy commands preloaded on every system's minimal installation       )"
var8="$(e.g: <grep> Um…what else…oh yeah!  The output of these is 1 or 0  🤓  )"

varlist='$var1 $var2 ... $var8'                # ←I assume 'this' would 
                                               #  prevent it from being expanded

for ENTRY in $varlist
do
  if [ "$ENTRY" ] # ←Already a boolean
  then
    <perform operation>
  else
    <log/echo it wasn't needed or something>
  fi
done

Would that work? Or rather — is it portable? I'm jumping from macOS*, Fedora, RHEL, Debian and "appliance" FreeBSD (pfSense, OPNsense, …) nonstop. That said, I do mean Fedora, RHEL, Debian, macOS and FreeBSD, no their derivatives/downstreams/whatever so things are very predictable and stable. The most I stray from these systems is Debian on WSL and only because SSH on PowerShell is very glitchy. Am I breaking rules? Do I need to enclose variables in the variables variable (varlist, I know you just know but to avoid ambiguities) in something else? Maybe escape the $?

If you have completely different alternatives to suggest, I'm listening. I'm open for anything as long as it still is Bash.

---

*: pre-Catalina still with Bash by default.

Answer 1

instead of numbered variables, use an array:

entries=(
    "$(some long operation that has different results on different systems   )"
    "$(bc of escaping or slighly different variations on the basic set of    )"
    "$(UNIXy commands preloaded on every system's minimal installation       )"
    "$(e.g: <grep> Um…what else…oh yeah!  The output of these is 1 or 0  🤓  )"
)
for entry in "${entries[@]}"; do
    echo "this is entry: $entry"
done

Works with the default bash 3.2 that ships with MacOS

Trying to stringify a list of strings and then separate them is destined to fail: see http://mywiki.wooledge.org/BashFAQ/050

Answer 2

You should use the ${!name} syntax for variable indirection.

Bash uses the value of the variable formed from the rest of parameter as the name of the variable

So this would work:

    varnames='var1 var2 ... var8'
    for varname in $varnames
    do
        if [ "${!varname}" ]
        ...

As for portability, that will depend on the bash version in each of your environments.