Take the 2-minute tour ×
Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems.. It's 100% free, no registration required.

Here is my bash case:

First case, this is what I want to do "aliasing" var with myvarA:

myvarA="variableA"
varname="A"
eval varAlias=\$"myvar"$varname
echo $varAlias

Second case for array variable and looping its members, which is trivial:

myvarA=( "variableA1" "variableA2" )
for varItem in ${myvarA[@]}
do
    echo $varItem
done

Now somehow I need to use "aliasing" technique like example 1, but this time for array variable:

eval varAlias=\$"myvar"$varname
for varItem in ${varAlias[@]}
do
    echo $varItem
done

But for last case, only first member of myvarA is printed, which is eval evaluate to value of the variable, how should I do var array variable so eval is evaluate to the name of array variable not the value of the variable.

share|improve this question
    
I think what I meant by "aliasing" is should be "indirection" in bash –  uray Sep 2 '10 at 22:17

6 Answers 6

The simplest form for parameter expansion is: ${parameter}.
Using braces in confused case is better way.

Considering of possibilities of being included spaces in array of "myvarA", I think this would be the answer.

#!/bin/bash -x
myvarA=( "variable  A1" "variable  A2" )
varname="A"

eval varAlias=( '"${myvar'${varname}'[@]}"' )
eval varAlias=( \"\${myvar${varname}[@]}\" ) # both works
for varItem in "${varAlias[@]}" # double quote and `@' is needed
do
    echo "$varItem"
done
share|improve this answer

In your answer, varAlias isn't an array, so you can do for varItem in $varAlias which is just doing word splitting. Because of that if any of your original array elements include spaces, they will be treated as separate words.

You can do scalar indirection like this: a=42; b=a; echo ${!b}.

You can do indirection to an array of scalars like this:

$ j=42; k=55; m=99
$ a=(j k m)
$ echo ${!a[1]}
55

Unfortunately, there's no satisfactory way to do the type of array indirection you're trying to do. You should be able to rework your code so that no indirection is needed.

See also BashFAQ/006.

share|improve this answer
up vote 1 down vote accepted

I solved it; last example should be like this:

eval varAlias=\${"myvar"$varname[@]}
for varItem in ${varAlias[@]}
do
    echo $varItem
done
share|improve this answer

Just to note that that above accepted answer is not complete. To do an actual array assignment you are missing parenthesis around your original array otherwise you will still get a new array of size 1

What i mean is that the following:

eval varAlias=\${"myvar"$varname[@]}

should be changed to:

eval varAlias=(\${"myvar"$varname[@]})

You can validate this by taking both cases and running:

echo ${#varAlias[@]}

In the original case you will get 1 in parenthesis you will get the actual number or elements in the original array. In both cases we basically create a new array.

share|improve this answer

It looks like you're trying to indirectly reference an array from the index of another.

You might like to do something like:

arr_one[0]=arr_two[@]

From there you can do:

cmd "${!arr_one[0]}"

...to indirectly reference a full expansion of "${arr_two[@]}". As near as I can tell, there is no direct method of indexing further. For example "${!arr_one[0][1]}" doesn't work as I'd hope (at least, not in bash) but you can do "${!arr_one[0]1:1}" and similar to slice the expansion as you could any other array. The end result is something like the 2-dimensional array structure that some other, more capable shells offer.

share|improve this answer

This is how you would create a dynamically named variable (bash version < 4.3).

# Dynamically named array
my_variable_name="dyn_arr_names"
eval $my_variable_name=\(\)

# Adding by index to the array eg. dyn_arr_names[0]="bob"
eval $my_variable_name[0]="bob"

# Adding by pushing onto the array eg. dyn_arr_names+=(robert)
eval $my_variable_name+=\(robert\)

# Print value stored at index indirect
echo ${!my_variable_name[0]}

# Print value stored at index
eval echo \${$my_variable_name[0]}

# Get item count
eval echo \${#$my_variable_name[@]}

Below is a group of functions that can be used to manage dynamically named arrays (bash version < 4.3).

# Dynamically create an array by name
function arr() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
     # The following line can be replaced with 'declare -ag $1=\(\)'
     # Note: For some reason when using 'declare -ag $1' without the parentheses will make 'declare -p' fail
    eval $1=\(\)
}

# Insert incrementing by incrementing index eg. array+=(data)
function arr_insert() { 
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    eval $1[\$\(\(\${#${1}[@]}\)\)]=\$2
}

# Update an index by position
function arr_set() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    eval ${1}[${2}]=\${3}
}

# Get the array content ${array[@]}
function arr_get() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    eval echo \${${1}[@]}
}

# Get the value stored at a specific index eg. ${array[0]}  
function arr_at() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    [[ ! "$2" =~ ^(0|[-]?[1-9]+[0-9]*)$ ]] && { echo "Array index must be a number" 1>&2 ; return 1 ; }
    local v=$1
    local i=$2
    local max=$(eval echo \${\#${1}[@]})
    # Array has items and index is in range
    if [[ $max -gt 0 && $i -ge 0 && $i -lt $max ]]
    then 
        eval echo \${$v[$i]}
    fi
}

# Get the value stored at a specific index eg. ${array[0]}  
function arr_count() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable " 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    local v=${1}
    eval echo \${\#${1}[@]}
}



array_names=(bob jane dick)

for name in "${array_names[@]}"
do
    arr dyn_$name
done

echo "Arrays Created"
declare -a | grep "a dyn_"

# Insert three items per array
for name in "${array_names[@]}"
do
    echo "Inserting dyn_$name abc"
    arr_insert dyn_$name "abc"
    echo "Inserting dyn_$name def"
    arr_insert dyn_$name "def"
    echo "Inserting dyn_$name ghi"
    arr_insert dyn_$name "ghi"
done

for name in "${array_names[@]}"
do
    echo "Setting dyn_$name[0]=first"
    arr_set dyn_$name 0 "first"
    echo "Setting dyn_$name[2]=third"
    arr_set dyn_$name 2 "third"
done 

declare -a | grep "a dyn_"

for name in "${array_names[@]}"
do
    arr_get dyn_$name
done


for name in "${array_names[@]}"
do
    echo "Dumping dyn_$name by index"
    # Print by index
    for (( i=0 ; i < $(arr_count dyn_$name) ; i++ ))
    do
        echo "dyn_$name[$i]: $(arr_at dyn_$name $i)"

    done
done

for name in "${array_names[@]}"
do
    echo "Dumping dyn_$name"
    for n in $(arr_get dyn_$name)
    do
        echo $n
    done
done

Below is a group of functions that can be used to manage dynamically named arrays (bash version >= 4.3).

# Dynamically create an array by name
function arr() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -g -a $1=\(\)   
}

# Insert incrementing by incrementing index eg. array+=(data)
function arr_insert() { 
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    declare -n r=$1
    r[${#r[@]}]=$2
}

# Update an index by position
function arr_set() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    declare -n r=$1 
    r[$2]=$3
}

# Get the array content ${array[@]}
function arr_get() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    declare -n r=$1 
    echo ${r[@]}
}

# Get the value stored at a specific index eg. ${array[0]}  
function arr_at() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable" 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    [[ ! "$2" =~ ^(0|[-]?[1-9]+[0-9]*)$ ]] && { echo "Array index must be a number" 1>&2 ; return 1 ; }
    declare -n r=$1 
    local max=${#r[@]}
    # Array has items and index is in range
    if [[ $max -gt 0 && $i -ge 0 && $i -lt $max ]]
    then 
        echo ${r[$2]}
    fi
}

# Get the value stored at a specific index eg. ${array[0]}  
function arr_count() {
    [[ ! "$1" =~ ^[a-zA-Z_]+[a-zA-Z0-9_]*$ ]] && { echo "Invalid bash variable " 1>&2 ; return 1 ; }
    declare -p "$1" > /dev/null 2>&1
    [[ $? -eq 1 ]] && { echo "Bash variable [${1}] doesn't exist" 1>&2 ; return 1 ; }
    declare -n r=$1
    echo ${#r[@]}
}



array_names=(bob jane dick)

for name in "${array_names[@]}"
do
    arr dyn_$name
done

echo "Arrays Created"
declare -a | grep "a dyn_"

# Insert three items per array
for name in "${array_names[@]}"
do
    echo "Inserting dyn_$name abc"
    arr_insert dyn_$name "abc"
    echo "Inserting dyn_$name def"
    arr_insert dyn_$name "def"
    echo "Inserting dyn_$name ghi"
    arr_insert dyn_$name "ghi"
done

for name in "${array_names[@]}"
do
    echo "Setting dyn_$name[0]=first"
    arr_set dyn_$name 0 "first"
    echo "Setting dyn_$name[2]=third"
    arr_set dyn_$name 2 "third"
done 

declare -a | grep 'a dyn_'

for name in "${array_names[@]}"
do
    arr_get dyn_$name
done


for name in "${array_names[@]}"
do
    echo "Dumping dyn_$name by index"
    # Print by index
    for (( i=0 ; i < $(arr_count dyn_$name) ; i++ ))
    do
        echo "dyn_$name[$i]: $(arr_at dyn_$name $i)"

    done
done

for name in "${array_names[@]}"
do
    echo "Dumping dyn_$name"
    for n in $(arr_get dyn_$name)
    do
        echo $n
    done
done

For more details on these examples visit Getting Bashed by Dynamic Arrays by Ludvik Jerabek

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.