Bash – How to Use a Variable as Part of an Array Name

bashshell

I have two arrays:

arrayA=(1 2 3)
arrayB=(a b c)

and I want to print out one of them using a command line argument, i.e., without any if else.

I tried a few variations on the syntax with no success. I am wanting to do something like this:

ARG="$1"

echo ${array${ARG}[@]}

but I get a "bad substitution" error. How can I achieve this?

Best Answer

Try doing this :

$ arrayA=(1 2 3)
$ x=A
$ var=array$x[@]
$ echo ${!var}
1 2 3

NOTE

from man bash (parameter expansion) :

    ${parameter}
           The value of parameter is substituted.
 The braces are required when parameter is a positional parameter with
  more than one
digit, or when parameter is followed by a character which is not to be interpreted as part of its name.
* If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion. * The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.

Related Solutions

Bash – Count number of elements in bash array, where the name of the array is dynamic (i.e. stored in a variable)

you should handle that stuff in the index evals. and you can indirect through your indirection variable's indices if you make it an array.

a=(abc1 def2 ghi3 jkl4 mno5)
r=('a[c=${#a[@]}]' a\[i] a\[@])
for   i in   0 1 2 3 4 5
do    c=
      printf "<%s>\n" "${!r-${!r[i<c?1:2]}}"
      printf "\n\tindex is $i and count is $c\n\n"
done

<abc1>

    index is 0 and count is 5

<def2>

    index is 1 and count is 5

<ghi3>

    index is 2 and count is 5

<jkl4>

    index is 3 and count is 5

<mno5>

    index is 4 and count is 5

<abc1>
<def2>
<ghi3>
<jkl4>
<mno5>

    index is 5 and count is 5

Because bash's indices are 0-based, the total count of array objects will always work out to one more than than the highest set index, and so:

c=
echo "${a[c=${#a[@]}]-this index is unset}" "$c"

this index is unset 5

...the parameter expands out to the default word if any is provided.

If one is not provided:

c=
${!r}
echo "$c"

...there's no harm done.

In the loop I track an $index variable and check if it is at least as large as $count. When it is lesser I expand the $reference var to a[i] because it is a valid index, but when it is equal or greater I expand the $ref to the entire $array.

Here it is in a function:

ref_arr(){
    local    index=-1 count=
    local    ref=(   "$1[ count= \${#$1[@]}  ]"
                     "$1[ index ]"    "$1[ @ ]"
    )  &&    printf  "input array '%s' has '%d' members.\n" \
                     "$1"  "${!ref-${count:?invalid array name: "'$1'"}}"
    while    [ "$((index+=1))" -lt "$count"  ]
    do       printf  "$1[$index]  ==  '%s'\n"  "${!ref[1]}"
    done
}
some_array=(some "dumb
            stuff" 12345\'67890 "" \
          '$(kill my computer)')
ref_arr some_array
ref_arr '$(echo won'\''t work)'

input array 'some_array' has '5' members.
some_array[0]  ==  'some'
some_array[1]  ==  'dumb
                stuff'
some_array[2]  ==  '12345'67890'
some_array[3]  ==  ''
some_array[4]  ==  '$(kill my computer)'
bash: count: invalid array name: '$(echo won't work)'

Bash Arrays – Using Variables in Array Names

Some ideas:

A "parameter expansion" of a variable value (the ${...} part):
```
echo "arr$COUNTER[0] = ${arr$COUNTER[0]}"
```
will not work. You may get around by using eval (but I do not recommend it):
```
eval echo "arr$COUNTER[0] = \${arr$COUNTER[0]}"
```
That line could be written as this:
```
i="arr$COUNTER[0]"; echo "$i = ${!i}"
```
That is called indirection (the !) in Bash.
A similar issue happens with this line:
```
declare -a "arr$COUNTER=($field)"
```
Which should be split into two lines, and eval used:
```
declare -a "arr$COUNTER"
eval arr$COUNTER\=$ \$field $
```
Again, I do not recommend using eval (in this case).
As you are reading the whole file into the memory of the shell, we may as well use a simpler method to get all lines into an array:
```
readarray -t lines <"VALUES_FILE.txt"
```
That should be faster than calling awk for each line.

An script with all the above could be:

#!/bin/bash
valfile="VALUES_FILE.txt"

readarray -t lines <"$valfile"             ### read all lines in.

line_count="${#lines[@]}"
echo "Total number of lines $line_count"

for ((l=0;l<$line_count;l++)); do
    echo "Counter value is $l"             ### In which line are we?
    echo "Field = ${lines[l]}"             ### kepth only to help understanding.
    k="arr$l"                              ### Build the variable arr$COUNTER
    IFS=" " read -ra $k <<<"${lines[l]}"   ### Split into an array a line.
    eval max=\${#$k[@]}                    ### How many elements arr$COUNTER has?
    #echo "field $field and k=$k max=$max" ### Un-quote to "see" inside.
    for ((j=0;j<$max;j++)); do             ### for each element in the line.
        i="$k[$j]"; echo "$i = ${!i}"      ### echo it's value.
    done
done
echo "The End"
echo

However, still, AWK may be faster, if we could execute what you need in AWK.

A similar processing could be done in awk. Assuming the 6 values will be used as an IP (4 of them) and the other two are a number and an epoch time.

Just a very simple sample of an AWK script:

#!/bin/sh
valfile="VALUES_FILE.txt"
awk '
NF==6 { printf ( "IP: %s.%s.%s.%s\t",$1,$2,$3,$4)
        printf ( "number: %s\t",$5+2)
        printf ( "epoch: %s\t",$6)
        printf ( "\n" )
    }
' "$valfile"

Just make a new question with the details.

Best Answer

NOTE

Related Solutions

Bash – Count number of elements in bash array, where the name of the array is dynamic (i.e. stored in a variable)

Bash Arrays – Using Variables in Array Names

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