Diff for "BashFAQ/005"

How can I use array variables?

This answer assumes you have a basic understanding of what arrays are in the first place. If you're new to this kind of programming, you may wish to start with the guide's explanation. BASH and KornShell have one-dimensional arrays indexed by a numerical expression, e.g.:

•  # Bash
   host=(mickey minnie goofy)
   n=${#host[*]}
   for ((i=0;i<n;i++)); do
       echo "host number $i is ${host[i]}"
   done

The indexing always begins with 0, unless you specifically choose otherwise. The awkward expression ${#host[*]} or ${#host[@]} returns the number of elements for the array host. (We'll go into more detail on syntax below.)

Ksh93 and Bash 4.0 have Associative Arrays as well. These are not available in Bourne, POSIX, ksh88 or older bash shells.

POSIX and Bourne shells are not guaranteed to have arrays at all (although a POSIX shell on any particular system may have them, they are not in the standard).

BASH and Korn shell arrays are sparse. Elements may be added and deleted out of sequence.

•  # Bash/ksh
   arr[0]=0
   arr[1]=1
   arr[2]=2
   arr[42]="what was the question?"
   unset 'arr[2]'
   echo "${arr[*]}"
   # prints 0 1 what was the question?

You should try to write your code in such a way that it can handle sparse arrays, unless you know in advance that an array will never have holes.

1. Loading values into an array

Assigning one element at a time is simple, and portable:

•  # Bash/ksh
   arr[0]=0
   arr[42]='the answer'

It's possible to assign multiple values to an array at once, but the syntax differs across shells.

•  # Bash/ksh93
   array=(zero one two three four)
  
   # Korn
   set -A array -- zero one two three four

When initializing in this way, the first index will be 0.

You can also initialize an array using a glob (see also NullGlob):

•  # Bash/ksh93
   oggs=(*.ogg)
  
   # Korn
   set -A oggs -- *.ogg

or using a substitution of any kind:

•  # Bash
   words=($sentence)
   letters=({a..z})    # Bash 3.0 or higher
  
   # Korn
   set -A words -- $sentence

When the arrname=(...) syntax is used, any unquoted substitutions inside the parentheses undergo WordSplitting and glob expansion according to the regular shell rules. In the first example above, if any of the words in $sentence contain glob characters, filename expansion may occur.

set -f and set +f may be used to disable and re-enable glob expansion, respectively, so that words like * will not be expanded into filenames. In some scripts, set -f may be in effect already, and therefore running set +f may be undesirable. This is something you must manage properly yourself; there is no easy or elegant way to "store" the glob expansion switch setting and restore it later. (And don't try to say parsing the output of set -o is easy, because it's not.)

1.1. Loading lines from a file or stream

In bash 4, the mapfile command (also known as readarray) accomplishes this:

 # Bash 4
 mapfile -t lines < myfile

 # or
 mapfile -t lines < <(some command)

See ProcessSubstitution and FAQ #24 for more details on the <() syntax.

mapfile handles blank lines (it inserts them as empty array elements), and it also handles missing final newlines from the input stream. Both those things become problematic when reading data in other ways, as we shall see momentarily.

mapfile does have one serious drawback: it can only handle newlines as line terminators. It can't, for example, handle NUL-delimited files from find -print0.

In other shells, we might start out like this:

 # These examples only work with certain kinds of input files.

 # Bash
 set -f; O=$IFS IFS=$'\n' lines=($(< myfile)) IFS=$O; set +f

 # Korn
 set -f; O=$IFS IFS='
 '; set -A lines -- $(< myfile); IFS=$O; set +f

We use IFS (setting it to a newline) because we want each line of input to become an array element, not each word. (This particular syntax may have undesired results with blank lines of input; see below for alternatives.)

That's a literal newline (and nothing else) between the single quotes in the Korn example.

Relying on IFS WordSplitting can cause issues if you have repeated whitespace delimiters that you wanted to be treated as multiple delimiters; e.g., a file with blank lines will have repeated newline characters. If you wanted the blank lines to be stored as empty array elements, IFS's behavior will backfire on you; the blank lines will disappear.

The solution to that is to read the elements one at a time, in a loop. Remember that in most shells (including bash), the subcommands of a pipeline are executed in subshells, so you might need to use something like this:

•  # Bash
   unset arr i
   while read -r; do arr[i++]=$REPLY; done < yourfile
  
   # or
   while read -r; do arr[i++]=$REPLY; done < <(your command)

Rather than piping your command to a while read loop, which would cause the array to be set in a subshell -- not very useful in most cases.

Also noteworthy for BASH is the fact that inside the square brackets, i++ works as a C programmer would expect. The square brackets in an array reference force an ArithmeticExpression. (That shortcut does not work in ksh88.)

If your file or data stream might be missing its final delimiter (e.g. a text file that might be missing a closing newline), the final read command in the loop might "fail" (terminating the loop) but still contain data. There are a couple ways to work around that:

•  # Bash
   unset arr i
   while read -r; do arr[i++]=$REPLY; done < <(your command)
   # Append unterminated data line if there was one.
   [[ $REPLY ]] && arr[i++]=$REPLY

Some people prefer reading directly into the array, which works great if there's an unterminated line (since the array element is populated with the partial data before the exit status of read is checked). Unfortunately, this puts an empty element on the end of the array if the data stream is correctly terminated:

•  # Bash
   unset arr i
   while IFS= read -r 'arr[i++]'; do :; done < <(your command)
   # Remove trailing empty element, if any.
   if [[ ${arr[i-1]} = "" ]]; then unset 'arr[--i]'; fi

Whether you prefer to read too many and then have to remove one, or read too few and then have to add one, is a personal choice.

NOTE: it is necessary to quote the 'arr[i++]' passed to read, so that the square brackets aren't interpreted as globs. This is also true for other non-keyword builtins that take a subscripted variable name, such as let and unset.

If you are trying to deal with records that might have embedded newlines, you might be using an alternative delimiter such as the NUL character ( \0 ) to separate the records. In that case, you'll want to use the -d argument to read as well:

•  # Bash
   unset arr i
   while IFS= read -rd '' 'arr[i++]'; do :; done < <(find . -name '*.ugly' -print0)
   if [[ ${arr[i-1]} = "" ]]; then unset 'arr[--i]'; fi
  
   # or
   while read -rd ''; do arr[i++]=$REPLY; done < <(find . -name '*.ugly' -print0)
   [[ $REPLY ]] && arr[i++]=$REPLY

1.2. Appending to an existing array

If you wish to append data to an existing array, there are several approaches. The most flexible is to keep a separate index variable:

•  # Bash/ksh93
   arr[i++]="new item"

If you don't want to keep an index variable, but you happen to know that your array is not sparse, then you can use the highest existing index:

•  # Bash/ksh
   # This will FAIL if the array has holes (is sparse).
   arr[${#arr[*]}]="new item"

If you don't know whether your array is sparse or not, but you don't mind re-indexing the entire array (and also being very slow), then you can use:

•  # Bash
   arr=("${arr[@]}" "new item")
  
   # Ksh
   set -A arr -- "${arr[@]}" "new item"

If you're in bash 3.1 or higher, then you can use the += operator:

•  # Bash 3.1
   arr+=("new item")

NOTE: the parentheses are required, just as when assigning to an array. (Or you will end up appending to ${arr[0]} which $arr is a synonym for.)

For examples of using arrays to hold complex shell commands, see FAQ #50 and FAQ #40.

2. Retrieving values from an array

${#arr[*]} or ${#arr[@]} gives the number of elements in an array:

•  # Bash
   shopt -s nullglob
   oggs=(*.ogg)
   echo "There are ${#oggs[*]} Ogg files."

When accessing the number of elements, * is quicker than @ in BASH in our testing on Bash-3, and gives the same result. (man bash: Arrays: ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is * or @, the expansion is the number of elements in the array.)

Using array elements en masse is one of the key features of shell arrays. In exactly the same way that "$@" is expanded for positional parameters, "${arr[@]}" is expanded to a list of words, one array element per word. For example,

•  # Korn/Bash
   for x in "${arr[@]}"; do
     echo "next element is '$x'"
   done

This works even if the elements contain whitespace. You always end up with the same number of words as you have array elements.

If one simply wants to dump the full array, one element per line, this is the simplest approach:

•  # Bash/ksh
   printf "%s\n" "${arr[@]}"

For slightly more complex array-dumping, "${arr[*]}" will cause the elements to be concatenated together, with the first character of IFS (or a space if IFS isn't set) between them. As it happens, "$*" is expanded the same way for positional parameters.

•  # Bash
   arr=(x y z)
   IFS=/; echo "${arr[*]}"; unset IFS
   # prints x/y/z

Unfortunately, you can't put multiple characters in between array elements using that syntax. You would have to do something like this instead:

•  # Bash/ksh
   arr=(x y z)
   x=$(printf "%s<=>" "${arr[@]}")
   echo "${x%<=>}"    # Remove the extra <=> from the end.
   # prints x<=>y<=>z

BASH 3.0 added the ability to retrieve the list of index values in an array, rather than just iterating over the elements:

•  # Bash 3.0 or higher
   arr=(0 1 2 3) arr[42]='what was the question?'
   unset 'arr[2]'
   echo ${!arr[*]}
   # prints 0 1 3 42

Retrieving the indices is extremely important in certain kinds of tasks, such as maintaining parallel arrays with the same indices (a cheap way to mimic having an array of structs in a language with no struct):

•  # Bash 3.0 or higher
   unset file title artist i
   for f in ./*.mp3; do
     file[i]=$f
     title[i]=$(mp3info -p %t "$f")
     artist[i++]=$(mp3info -p %a "$f")
   done
  
   # Later, iterate over every song.
   # This works even if the arrays are spare, just so long as they all have
   # the SAME holes.
   for i in ${!file[*]}; do
     echo "${file[i]} is ${title[i]} by ${artist[i]}"
   done

3. Other tricks

Bash's Parameter Expansions may be performed on array elements en masse:

•  # Bash
   arr=(abc def ghi jkl)
   echo "${arr[@]#?}"          # prints bc ef hi kl
   echo "${arr[@]/[aeiou]/}"   # prints bc df gh jkl

Parameter Expansion can also be used to extract elements from an array. Some people call this slicing:

•  # Bash
   echo "${arr[@]:1:3}"        # three elements starting at #1 (second element)
   echo "${arr[@]:(-2)}"       # last two elements
   echo "${@:(-1)}"            # last positional parameter
   echo "${@:(-2):1}"          # second-to-last positional parameter

As we see above, the @ array (the array of positional parameters) can be used almost like a regularly named array. This is the only array available for use in POSIX or Bourne shells. It has certain limitations: you cannot individually set or unset single elements, and it cannot be sparse. Nevertheless, it still makes certain POSIX shell tasks possible that would otherwise require external tools:

•  # POSIX
   set -- *.mp3
   if [ -e "$1" ]; then
     echo "there are $# MP3 files"
   else
     echo "there are 0 MP3 files"
   fi

   # POSIX
   ...
   # Add an option to our dynamically generated list of options
   set -- "$@" -f "$somefile"
   ...
   foocommand "$@"

(Compare to FAQ #50's dynamically generated commands using named arrays.)

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