Bash variable variables passed through functions - bash

Question

I've looked over a couple other variable variable posts here but still seem stuck with what I'm trying to attempt.
I have an existing script that has a series of similar block like
set_name="something"
# Required values
var1=$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "key1_${set_name}")
if [[ -z "$var1" ]]; then
echo "Cannot find required var1"
exit 1
fi
var2=$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "key2_${set_name}")
if [[ -z "$var2" ]]; then
echo "Cannot find required var2"
exit 1
fi
# Optional, okay to be empty
var3=$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "key3_${set_name}")
var4=$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "key4_${set_name}")
I was trying to factor some of the boilerplate checks out to keep this set of lookups assignment easier to read (in my opinion anyway). The last iteration I had attempted (clearly not working) looks like
ZTest () {
var=$1
if [[ -z "${!var}" ]]; then
echo $2
exit 1
fi
}
VarRequire () {
var=$1
key=$2
errmsg=$3
VarLookup ${!var} $key
ZTest ${!var} $errmsg
}
VarLookup () {
var=$1
key=$2
${!var}=$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "$key")
}
# Required
VarRequire "var1" "key1_${set_name}" "Cannot find required var1"
VarRequire "var2" "key2_${set_name}" "Cannot find required var2"
# optional
VarLookup "var3" "key3_${set_name}"
VarLookup "var4" "key4_${set_name}"
The end result is I would be able to reference $var1, $var2, $var3, $var4 down the line in the script just the same as the original.
Is what I'm attempting possible in bash?

Answer 1

ZTest is already available as a parameter expansion operator:
: ${var1:?Cannot find required var1}
You are close with VarLookup, though; you need to use the declare command to create the variable. ${!var} is only for accessing the value once the variable exists. (Note that declare requires the -g option to avoid creating a local variable, and that option was only introduced in version 4.2.)
VarLookup () {
local var=$1
local key=$2
declare -g "${var}=$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "$key")"
}
Prior to version 4.2, you can use printf in place of declare:
printf -v "$var" '%s' "$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "$key")"
I would resist the urge to refactor too much in shell, as indirection can be fragile. I'd suggest something more direct like
var_lookup () {
REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "${1}_$set_name"
}
var1=$(var_lookup key1); : ${var1:?Cannot find required var1}
var2=$(var_lookup key2); : ${var2:?Cannot find required var2}
var3=$(var_lookup key3)
var4=$(var_lookup key4)

Answer 2

Too much indirection in the Var* functions
VarRequire () {
local var=$1
local key=$2
local errmsg=$3
VarLookup "$var" "$key"
ZTest "$var" "$errmsg"
}
VarLookup () {
local var=$1
local key=$2
declare -g "$var"="$(REQUIRED_ENV_VAR=/path/to/somewhere mybinary -p "$key")"
}
The declare command allows you to use a variable's value as the variable name. I use the -g option so the variable is global.
The ZTest function does require the indirection.

Answer 3

From the BASH manpage
local [option] [name[=value] ...]
For each argument, a local variable named name is created,
and assigned value. The option can be any of the options
accepted by declare. When local is used within a function, it
causes the variable name to have a visible scope restricted to
that function and its children. With no operands, local writes a
list of local variables to the standard output. It is an
error to use local when not within a function. The return status
is 0 unless local is used outside a function, an invalid name is
supplied, or name is a readonly variable.
So the answer is yes. If you declare a variable a local within a function, and it calls another function, that function has access to the variable as well.

Answer 4

Can you set REQUIRED_ENV_VAR=/path/to/somewhere once at the start?
And have your mybinary return 0 when a var is found?
You might like something like
function mylog {
errortype=$1
shift
echo "(maybe show ${errortype}) $*"
if [ "$errortype" = "error" ]; then
exit 1
fi
}
var1=$(mybinary -p "key1_${set_name}") || mylog error "Cannot find required var1"
var2=$(mybinary -p "key2_${set_name}") || mylog error "Cannot find required var2"
var3=$(mybinary -p "key3_${set_name}")
var4=$(mybinary -p "key4_${set_name}") || mylog debug "Just want to say something"