In the shell I can do

$ a=b b=c; eval echo \$$a
c

How do I do the same with GNU Make's $(eval) function?

$ cat Makefile
a=b
b=c
x:; echo {What goes here in order to get:}
$ make
c

Sorry, that was all too confusing, (my bad) Let me try to explain again, but with a more simple example:

#define a new var and export it to the "env":
export VAR_ONE=thisIsVarOne
# check if it was created:
env | grep VAR_ONE      #this displays: "VAR_ONE=thisIsVarOne"
# use it in a simple echo command:
echo VAR_ONE=$VAR_ONE  #this displays: "VAR_ONE=thisIsVarOne"

# create a new var with the name that contains the value of VAR_ONE ("newvarthisIsVarOne") in its name:
export newvar${VAR_ONE}="somePrefix${VAR_ONE}"  
# verify if it has been created:
env | grep newvar  # this displays: "newvarthisIsVarOne=somePrefixthisIsVarOne"

### Now my question: How do I use that new, syntetic, variable? Please note that: env | grep newvar shows that there has been created a variable with the name newvar${VAR_ONE} But I can't **address** that new variable, e.g. in an echo command. I tried: echo ${newvar${VAR_ONE}} But that gave me bash: ${newvar${VAR_ONE}}: bad substitution ### Constraints: 1. The value of VAR_ONE can change, so I can't use that to address the newly created variable 2. I can't declare a new variable with a static name. For usage, further in the script, I need the keyword VAR_ONE as part of the name of the newly created variable. The name of the variable needs to be something like: newvar${VAR_ONE} Thank you in advance ChriV

One challenging thing I've encountered in bash scripting is using variable expansion in a recursive way. However, I was able to figure out [through ChatGPT] that I could use indirect subsitution to reference positional parameter values:

linkerArgs+=("${!i}")

However, if I am using string substitution, it's fine for me to just use expansion in a recursive manner as you would figure logically:

"${x// /$newChar}"

Why does the bash interpreter consider the latter type of expansion (with just the dollar sign inside the brackets) as "bad substitution" when used in the context of assigning items to an array, but not with string substitution?

So I know that you can test for the existence of a regular parameter via indirect expansion by doing something like: foo=1 bar=foo (( ${(P)+bar} )) && print "$bar exists" And I know you can test for the existence of a key inside an associative array by doing something like: foo=([abc]=1) (( ${+foo[abc]} )) && print "abc exists" However I can't figure out how to combine the two and test for the existence of a key inside an associative array via indirect expansion. Is this possible without using eval? I tried several combinations including the following, and none of them worked: foo=([abc]=1) bar=foo (( ${(P)+bar[abc]} )) && print "$bar has key abc" # Test fails (( ${(P)+${bar}[abc]} )) && print "$bar has key abc" # Passes for nonexistant keys (( ${${(P)+bar}[abc]} )) && print "$bar has key abc" # Test fails (( ${${(P)bar}+[abc]} )) && print "$bar has key abc" # prints "zsh: bad output format specification"

#!/bin/bash
mat_1=(ServerAB ServerFR ServerPE ServerAM ServerHU)
st="mat_1";
indirect_var='${'${st}'[@]}'

#(Please, see the "--Desired Ouput Section--" in comments)

#----- What is Hapenning now at output ----
echo Values of "mat_1 ": ${mat_1[@]}
echo Indirect value of "mat_1": ${!indirect_var}

# echo Indirect value of "mat_1": ${!indirect_var}  ##output> ${mat_1[@]}
# But it is not recognized as a real ${mat_1[@]}.

# -- What actually have ----
for (( i=0; i  function-name $1, where $1 is
                                     # an string, and this string already
                                     # exist above , it will interpret as
                                     # an existing array
 

do
   echo ${mat_1[i]};
done

#And I would like those strings 
#that are part of the name of existing 
#variables , will be treated as an input
# and this loop works. I will show What I have done,
# what I have reached, and what I expect to have.
# Thank you!


#------What I expect works-----

#for (( i=0; i < ${#$st[@]}; i++ ))  #I would like $st works like 'mat_1'
                                     #and this loop can be run without 
                                     #problems
#do
#   echo ${$st[i]};
#done

#--- Actual Output ------------
#$ ./matrix.sh 
# Values of mat_1 : ServerAB ServerFR ServerPE ServerAM ServerHU
# ./matrix.sh: line 8: ${mat_1[@]}: invalid variable name
# ServerAB
# ServerFR
# ServerPE

#--- Desired Output ----------

#$ ./matrix.sh 
# Values of mat_1 : ServerAB ServerFR ServerPE ServerAM ServerHU
# Indirect Value of mat_1: ServerAB ServerFR ServerPE ServerAM ServerHU
# ServerAB
# ServerFR
# ServerPE
# ServerAM
# ServerHU

" Hi Friends, I would like some ideas to achieve the following". - I have many existing 'arrays vars' that I would like to be called within a for loop by 'concatenating strings' in order to form those ' existing array names'.But in the Script above I just only Working with 01 array 'var_mat1'. I jut only need work for 01 array... - Example of the existing 'array names': var_mat1=( ".." ".." ".." ) var_mat2=( ".." ".." ".." ) . . . var_matN=( ".." ".." ".." )

Say:

variable="Something that it holds"

then echo "$variable" will output: Something that it holds But say I also do:

var2="variable";  
echo "\$$(echo $var2)"

will just output: $variable And not: Something that it holds Can anyone tell me about what feature of Unix is in play, here?

I am trying to do the following indirect task:

host_1=(192.168.0.100 user1 pass1)
host_2=(192.168.0.101 user2 pass2)

hostlist=( "host_1" "host_2" )

for item in ${hostlist[@]}; do

current_host_ip=${!item}
current_host_user=${!item}
current_host_pass=${!item}

echo "IP: $current_host_ip User: $current_host_user Pass: $current_host_pass"

done

I'm trying to understand how should I perform this indirect request so I pull the hostname from the array "hostlist", and then I should do indirect request to pull the host 1 IP, user and pass. But when I'm trying to do it, I'm stuck with either only first variable (only IP), or all variables inside one (if I add [@] into the end of variable name), empty result, or numbers from array. I can't understand how can I first copy the host_1 array into current_ variables and then (after my script does some work) I need to pass the host_2 variables to the same variables current_. Can you pinpoint my mistake? I think this is the solution to the problem I just can't adopt it: https://unix.stackexchange.com/questions/20171/indirect-return-of-all-elements-in-an-array

In bash, I understand we can have variable indirect expansion via two ways: - Using declare: declare -n foo=bar - Using the ${!..} expansion. We can combine both: declare -n foo=SHELL bar=foo echo ${!bar} gives: /bin/bash Is it possible to extend this to more levels? --- It's mostly as for writing obfuscated code - some of my friends are challenging each other.

Consider the following example, it seems it's working fine with the index 0: $ a1=(1 2 3) $ a2=(a b c) $ for x in a1 a2; do echo "${!x}"; done 1 a $ for x in a1 a2; do echo "${!x}"; done 1 a However with the index 1 it prints nothing: $ for x in a1 a2; do echo "${!x}"; done Arrays just by themselves are fine: $ echo "${a1} ${a2}" 2 b ### Edit - A real life use case based on *ilkkachu* answer SHIBB=(https://shibboleth.net/downloads/service-provider/3.0.2/ shibboleth-sp-3.0.2 .tar.gz) XERCES=(http://apache.mirrors.nublue.co.uk//xerces/c/3/sources/ xerces-c-3.2.1 .tar.gz) XMLSEC=(http://apache.mirror.anlx.net/santuario/c-library/ xml-security-c-2.0.1 .tar.gz) XMLTOOL=(http://shibboleth.net/downloads/c++-opensaml/latest/ xmltooling-3.0.2 .tar.gz) OPENSAML=(http://shibboleth.net/downloads/c++-opensaml/latest/ opensaml-3.0.0 .tar.gz) typeset -n x for x in XERCES XMLSEC XMLTOOL OPENSAML SHIBB; do url="${x}" app="${x}" ext="${x}" [ -f "./${app}${ext}" ] || wget "${url}${app}${ext}" tar -xf "./${app}${ext}" cd "./${app}" && ./configure && make -j2 && make install && ldconfig cd .. done