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

问题:

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?

回答1:

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.
• $(...) - Command Subsitution
• Aside: tr ' ' '\n' <<< "${ids[@]}" is a more efficient way of doing: echo "${ids[@]}" | tr ' ' '\n'

回答2:

If you're running Bash version 4 or above (which should be the case in any modern version of Linux), you can get unique array values in bash by creating a new associative array that contains each of the values of the original array. Something like this:

$ a=(aa ac aa ad "ac ad")
$ declare -A b
$ for i in "${a[@]}"; do b["$i"]=1; done
$ printf '%s\n' "${!b[@]}"
ac ad
ac
aa
ad

This works because in an array, each key can only appear once. When the for loop arrives at the second value of aa in a[2], it overwrites b[aa] which was set originally for a[0].

Doing things in native bash can be faster than using pipes and external tools like sort and uniq.

If you're feeling confident, you can avoid the for loop by using printf's ability to recycle its format for multiple arguments, though this seems to require eval. (Stop reading now if you're fine with that.)

$ eval b=( $(printf ' ["%s"]=1' "${a[@]}") )
$ declare -p b
declare -A b=(["ac ad"]="1" [ac]="1" [aa]="1" [ad]="1" )

The reason this solution requires eval is that array values are determined before word splitting. That means that the output of the command substitution is considered a single word rather than a set of key=value pairs.

While this uses a subshell, it uses only bash builtins to process the array values. Be sure to evaluate your use of eval with a critical eye. If you're not 100% confident that chepner or glenn jackman or greycat would find no fault with your code, use the for loop instead.

回答3:

If your array elements have white space or any other shell special character (and can you be sure they don't?) then to capture those first of all (and you should just always do this) express your array in double quotes! e.g. "${a[@]}". Bash will literally interpret this as "each array element in a separate argument". Within bash this simply always works, always.

Then, to get a sorted (and unique) array, we have to convert it to a format sort understands and be able to convert it back into bash array elements. This is the best I've come up with:

eval a=($(printf "%q\n" "${a[@]}" | sort -u))

Unfortunately, this fails in the special case of the empty array, turning the empty array into an array of 1 empty element (because printf had 0 arguments but still prints as though it had one empty argument - see explanation). So you have to catch that in an if or something.

Explanation: The %q format for printf "shell escapes" the printed argument, in just such a way as bash can recover in something like eval! Because each element is printed shell escaped on it's own line, the only separator between elements is the newline, and the array assignment takes each line as an element, parsing the escaped values into literal text.

e.g.

> a=("foo bar" baz)
> printf "%q\n" "${a[@]}"
'foo bar'
baz
> printf "%q\n"
''

The eval is necessary to strip the escaping off each value going back into the array.

回答4:

I realize this was already answered, but it showed up pretty high in search results, and it might help someone.

printf "%s\n" "${IDS[@]}" | sort -u

Example:

~> IDS=( "aa" "ab" "aa" "ac" "aa" "ad" )
~> echo  "${IDS[@]}"
aa ab aa ac aa ad
~>
~> printf "%s\n" "${IDS[@]}" | sort -u
aa
ab
ac
ad
~> UNIQ_IDS=($(printf "%s\n" "${IDS[@]}" | sort -u))
~> echo "${UNIQ_IDS[@]}"
aa ab ac ad
~>

回答5:

'sort' can be used to order the output of a for-loop:

for i in ${ids[@]}; do echo $i; done | sort

and eliminate duplicates with "-u":

for i in ${ids[@]}; do echo $i; done | sort -u

Finally you can just overwrite your array with the unique elements:

ids=( `for i in ${ids[@]}; do echo $i; done | sort -u` )

回答6:

this one will also preserve order:

echo ${ARRAY[@]} | tr [:space:] '\n' | awk '!a[$0]++'

and to modify the original array with the unique values:

ARRAY=($(echo ${ARRAY[@]} | tr [:space:] '\n' | awk '!a[$0]++'))

回答7:

To create a new array consisting of unique values, ensure your array is not empty then do one of the following:

Remove duplicate entries (with sorting)

readarray -t NewArray < <(printf '%s\n' "${OriginalArray[@]}" | sort -u)

Remove duplicate entries (without sorting)

readarray -t NewArray < <(printf '%s\n' "${OriginalArray[@]}" | awk '!x[$0]++')

Warning: Do not try to do something like NewArray=( $(printf '%s\n' "${OriginalArray[@]}" | sort -u) ). It will break on spaces.

回答8:

If you want a solution that only uses bash internals, you can set the values as keys in an associative array, and then extract the keys:

declare -A uniqs
list=(foo bar bar "bar none")
for f in "${list[@]}"; do 
  uniqs["${f}"]=""
done

for thing in "${!uniqs[@]}"; do
  echo "${thing}"
done

This will output

bar
foo
bar none

回答9:

cat number.txt

1 2 3 4 4 3 2 5 6

print line into column: cat number.txt | awk '{for(i=1;i<=NF;i++) print $i}'

1
2
3
4
4
3
2
5
6

find the duplicate records: cat number.txt | awk '{for(i=1;i<=NF;i++) print $i}' |awk 'x[$0]++'

4
3
2

Replace duplicate records: cat number.txt | awk '{for(i=1;i<=NF;i++) print $i}' |awk '!x[$0]++'

1
2
3
4
5
6

Find only Uniq records: cat number.txt | awk '{for(i=1;i<=NF;i++) print $i|"sort|uniq -u"}

1
5
6

回答10:

Without loosing the original ordering:

uniques=($(tr ' ' '\n' <<<"${original[@]}" | awk '!u[$0]++' | tr '\n' ' '))

回答11:

Try this to get uniq values for first column in file

awk -F, '{a[$1];}END{for (i in a)print i;}'