Bash shell Function that Reads a file to an array

Question

I am writing a shell script function that reads a given file and store the data into a 2D array.

File.txt

a1 b1 c1
a2 b2 c2
a3 b3 c3
# Let's just assume 3 rows

The idea is to pass the storage by reference using eval, with the function signature read_data File.txt array num_rows

read_data() {

  if [ ! -f $1 ]; then
    echo "Failed to read hosts ($1 not found)"; 
    exit;
  fi  

  while read -r line; do

    # skip the comments
    { [[ "$line" =~ ^#.*$ ]] || [[ -z $line ]] ;} && continue

    # Parse the line
    read -a tokens <<< $line

    if [ ${#tokens[@]} == 3 ]; then

      # Extract the user/host/home
      eval $2[\$$3, 0]=\${tokens[0]}
      eval $2[\$$3, 1]=\${tokens[1]}
      eval $2[\$$3, 2]=\${tokens[2]}
      eval $3=$(($3+1))

    else
      echo "Wrong line format '$line'"
    fi  

  done < $1
}

After calling below

declare -a array
num_rows=0
read_data File.txt array num_rows

What I get is num_rows equals to 3 but the contents stored in array is

pnt_data() {
  for ((i=0; i<$2; i++)); do

      eval a=\${$1[$i, 0]} 
      eval b=\${$1[$i, 1]} 
      eval c=\${$1[$i, 2]} 

      echo $a $b $c
  done
}

pnt_data array num_rows

a3 b3 c3
a3 b3 c3
a3 b3 c3

What happen to this? Is there anything wrong with my function?

Bash does not support 2D arrays, choose a different language. awk kinda supports them. Personally I would consider Perl or Python. — cdarke
– cdarke, Commented Aug 17, 2016 at 18:37

chepner · Accepted Answer · 2016-08-17 17:52:24Z

4

Array indices are evaluated in an arithmetic context, and the comma operator works by evaluating the left-hand operand, ignoring it, then evaluating to the right-hand operand. For example:

$ echo $((3,0))
0
$ echo $((3+9,0))
0
$ echo $((a,0))
0

As a result, all of the following assignments are equivalent to foo[0]=bar:

foo[3,0]=bar     # ignore 3
foo[3+9,0]=bar   # 3+9=12, ignore 12
foo[a,0]=bar     # ignore whatever the value of a is

If you want to simulate multidimensional indices, you'll need to use an associative array, so that the string used as an index does not undergo arithmetic expansion before using it.

declare -A array  # capital A, not lowercase a
num_rows=0
read_data File.txt array num_rows

edited Aug 17, 2016 at 17:52

answered Aug 17, 2016 at 17:37

chepner

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1 Comment

Jes Over a year ago

can u elaborate it more?

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