How can I get unique values from an array in Bash?

Jetse picture Jetse · Nov 30, 2012 · Viewed 90.7k times · Source

I've got almost the same question as here.

I have an array which contains aa ab aa ac aa ad, etc. Now I want to select all unique elements from this array. Thought, this would be simple with sort | uniq or with sort -u as they mentioned in that other question, but nothing changed in the array... The code is:

echo `echo "${ids[@]}" | sort | uniq`

What am I doing wrong?

Answer

sampson-chen picture sampson-chen · Nov 30, 2012

A bit hacky, but this should do it:

echo "${ids[@]}" | tr ' ' '\n' | sort -u | tr '\n' ' '

To save the sorted unique results back into an array, do Array assignment:

sorted_unique_ids=($(echo "${ids[@]}" | tr ' ' '\n' | sort -u | tr '\n' ' '))

If your shell supports herestrings (bash should), you can spare an echo process by altering it to:

tr ' ' '\n' <<< "${ids[@]}" | sort -u | tr '\n' ' '

Input:

ids=(aa ab aa ac aa ad)

Output:

aa ab ac ad

Explanation:

  • "${ids[@]}" - Syntax for working with shell arrays, whether used as part of echo or a herestring. The @ part means "all elements in the array"
  • tr ' ' '\n' - Convert all spaces to newlines. Because your array is seen by shell as elements on a single line, separated by spaces; and because sort expects input to be on separate lines.
  • sort -u - sort and retain only unique elements
  • tr '\n' ' ' - convert the newlines we added in earlier back to spaces.
  • $(...) - Command Substitution
  • Aside: tr ' ' '\n' <<< "${ids[@]}" is a more efficient way of doing: echo "${ids[@]}" | tr ' ' '\n'