Update array passed by reference with BASH

Question

I would like to write a function that takes an array variable name and updates the contents. For example:

ARRAY1=("test 1" "test 2" "test 3")
toUpper ARRAY1

for arg in "${ARRAY1[@]}"; do
  echo "arg=$arg"
done

# output
arg=TEST 1
arg=TEST 2
arg=TEST 3

I have a crude attempt at doing this which requires a copy of the input array. Using indirect references, I am able to create a copy of the input variable. The copy of the array is used to get the count of the elements. If there is a better way to do this please let me know.

function toUpper() {
  local ARRAY_NAME=$1
  local ARRAY_REF="$ARRAY_NAME[@]"
  # use an indirect reference to copy the array so we can get the count
  declare -a ARRAY=("${!ARRAY_REF}")

  local COUNT=${#ARRAY[@]}

  for ((i=0; i<$COUNT; i++)); do
    local VAL="${ARRAY[$i]}"
    VAL=$(echo $VAL | tr [:lower:] [:upper:])
    echo "ARRAY[$i]=\"$VAL\""
    eval "$ARRAY_NAME[$i]=\"$VAL\""
  done
}

ARRAY1=( "test" "test 1" "test 3" )

toUpper ARRAY1

echo
echo "Printing array contents"
for arg in "${ARRAY1[@]}"; do
  echo "arg=$arg"
done

Using BASH 4 but would be nice if the solution would work with version 3 if possible. — dcompiled
– dcompiled, Commented Apr 6, 2015 at 16:27
BTW, all-uppercase variable names for non-exported shell variables is bad form. Per the POSIX spec (pubs.opengroup.org/onlinepubs/009695399/basedefs/…): "The name space of environment variable names containing lowercase letters is reserved for applications"; since environment and local variables share a namespace, using lower-case names prevents overwriting environment variables and builtins by accident. — Charles Duffy
– Charles Duffy, Commented Apr 6, 2015 at 17:11
As an aside -- the function keyword is best avoided. It has no semantic use -- function foo() { ...; } has precisely the same effect as foo() { ...; } in bash -- but needlessly makes code incompatible with POSIX sh. — Charles Duffy
– Charles Duffy, Commented Apr 6, 2015 at 22:03

Charles Duffy · Accepted Answer · 2015-04-06 22:01:15Z

6

Using BASH 4.3+ you can do

arr=( "test" "test 1" "test 3" )
toUpper() { declare -n tmp="$1"; printf "%s\n" "${tmp[@]^^}"; }

toUpper arr
TEST
TEST 1
TEST 3

Update: To reflect the changes in original array:

toUpper() {
   declare -n tmp="$1"; 
   for ((i=0; i<"${#tmp[@]}"; i++)); do
      tmp[i]="${tmp[i]^^}"
    done;
}

arr=( "test" "test 1" "test 3" )
toUpper arr
printf "%s\n" "${arr[@]}"
TEST
TEST 1
TEST 3

Update2: Here is a way to make it work in older BASH (prior to 4) versions without eval:

upper() {
   len=$2
   for ((i=0; i<len; i++)); do
      elem="${1}[$i]"
      val=$(tr '[:lower:]' '[:upper:]' <<< "${!elem}")
      IFS= read -d '' -r "${1}[$i]" < <(printf '%s\0' "$val")
   done;
}

arr=( "test" "test 1" "test 3" )
upper arr ${#arr[@]}
printf "%s\n" "${arr[@]}"
TEST
TEST 1
TEST 3

edited Apr 6, 2015 at 22:01

Charles Duffy

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answered Apr 6, 2015 at 14:43

anubhava

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16 Comments

Charles Duffy Over a year ago

Not just 4+, but 4.3+; that said, when targeting a new enough bash, this is certainly the best choice.

anubhava Over a year ago

I believe it is declare -n (reference variable) that was added in 4.3

Charles Duffy Over a year ago

I'd suggest using read -r if not wanting to have side effects on the data (expanding backslash escape sequences).

Charles Duffy Over a year ago

Quoting [:lower:] and [:upper:] may also be appropriate, to prevent those strings from being glob-matched before being passed to tr. I note that I made the same mistake there myself.

Charles Duffy Over a year ago

@anubhava, if we don't want to truncate at the first newline in the string. IFS controls characters used for trimming words, and characters considered whitespace to be trimmed from the end; -d controls the characters which ends the content read by a single invocation of read. Compare IFS= read -r <<<$'one\ntwo'; printf '%q\n' "$REPLY" and IFS= read -r -d '' <<<$'one\ntwo'; printf '%q\n' "${REPLY%$'\n'}".

|

Charles Duffy · Accepted Answer · 2015-04-06 20:48:26Z

1

anubhava's answer is ideal for bash 4.3 or newer. To support bash 3, one can use eval (very cautiously, with strings generated with printf %q) to replace the use of namevars, and tr to replace the ${foo^^} expansion for upper case:

toUpper() {
  declare -a indexes
  local cmd idx item result
  printf -v cmd 'indexes=( "${!%q[@]}" )' "$1"; eval "$cmd"
  for idx in "${indexes[@]}"; do
     printf -v cmd 'item=${%q[%q]}' "$1" "$idx"; eval "$cmd"
     result=$(tr '[:lower:]' '[:upper:]' <<<"$item")
     printf -v cmd '%q[%q]=%q' "$1" "$idx" "$result"; eval "$cmd"
  done
}

edited Apr 6, 2015 at 20:48

answered Apr 6, 2015 at 17:17

Charles Duffy

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1 Comment

Charles Duffy Over a year ago

(Notably, in newer versions of bash, printf -v can assign directly to an array element; however, that's a 4.2 feature, not available in 3.x -- but still can be helpful if one isn't quite running 4.3).

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