#pragma section-numbers 2
<<Anchor(faq5)>>
== How can I use array variables? ==
This answer assumes you have a basic understanding of what arrays ''are''.  If you're new to this kind of programming, you may wish to start with [[BashGuide/Arrays|the guide's explanation]].  This page is more thorough. See [[#See_Also|links]] at the bottom for more resources.

<<TableOfContents>>

=== Intro ===
One-dimensional integer-indexed arrays are implemented by Bash, Zsh, and most KornShell varieties including AT&T ksh88 or later, mksh, and pdksh. Arrays are not specified by POSIX and not available in legacy or minimalist shells such as BourneShell and Dash. The POSIX-compatible shells that do feature arrays mostly agree on their basic principles, but there are some significant differences in the details. Advanced users of multiple shells should be sure to research the specifics. Ksh93, Zsh, and Bash 4.0 additionally have [[BashGuide/Arrays#Associative_Arrays|Associative Arrays]] (see also [[BashFAQ/006|FAQ 6]]). This article focuses on indexed arrays as they are the most common type.

Basic syntax summary (for bash, math indexed arrays):
||`a=(word1 word2 "$word3" ...)`||Initialize an array from a word list, indexed starting with 0 unless otherwise specified.||
||`a=(*.png *.jpg)`||Initialize an array with filenames.||
||`a[i]=word`||Set one element to `word`, evaluating the value of `i` in a math context to determine the index.||
||`a[i+1]=word`||Set one element, demonstrating that the index is also a math context.||
||`a[i]+=suffix`||Append `suffix` to the previous value of `a[i]` (bash 3.1).||
||`a+=(word ...)` # append||<|2> Modify an existing array without unsetting it, indexed starting at one greater than the highest indexed element unless otherwise specified (bash 3.1).||
||`a+=([3]=word3 word4 [i]+=word_i_suffix)`<<BR>># modify (ormaaj example)||
||`unset 'a[i]'`||Unset one element. Note the mandatory quotes (`a[i]` is a valid [[glob]]).||
||`"${a[i]}"`||Reference one element.||
||`"$(( a[i] + 5 ))"`||Reference one element, in a math context.||
||`"${a[@]}"`||Expand all elements as a list of words.||
||`"${!a[@]}"`||Expand all ''indices'' as a list of words (bash 3.0).||
||`"${a[*]}"`||Expand all elements as a ''single'' word, with the first char of [[IFS]] as separator.||
||`"${#a[@]}"`||Number of elements (size, length).||
||`"${a[@]:start:len}"`||Expand a range of elements as a list of words, cf. [[BashFAQ/100#Extracting_parts_of_strings|string range]].||
||`"${a[@]#trimstart}"` `"${a[@]%trimend}"`<<BR>>`"${a[@]//search/repl}"` etc.||Expand all elements as a list of words, with modifications applied to each element separately.||
||`declare -p a`||Show/dump the array, in a bash-reusable form.||
||`mapfile -t a < stream`||Initialize an array from a stream (bash 4.0).||
||`readarray -t a < stream`||Same as mapfile.||
||`"$a"`||Same as `"${a[0]}"`. '''Does NOT''' expand to the entire array. This usage is considered '''confusing''' at best, but is usually a '''bug'''.||

Here is a typical usage pattern featuring an array named `host`:

{{{#!highlight bash
# Bash

# Assign the values "mickey", "minnie", and "goofy" to sequential indexes starting with zero.
host=(mickey minnie goofy)

# Iterate over the indexes of "host".
for idx in "${!host[@]}"; do
    printf 'Host number %d is %s\n' "$idx" "${host[idx]}"
done
}}}

`"${!host[@]}"` expands to the indices of of the `host` array, each as a separate word.

Indexed arrays are ''sparse'', and elements may be inserted and deleted out of sequence.

{{{#!highlight bash
# Bash/ksh

# Simple assignment syntax.
arr[0]=0
arr[2]=2
arr[1]=1
arr[42]='what was the question?'

# Unset the second element of "arr"
unset -v 'arr[2]'

# Concatenate the values, to a single argument separated by spaces, and echo the result.
echo "${arr[*]}"
# outputs: "0 1 what was the question?"
}}}

It is good practice to write your code in such a way that it can handle sparse arrays, even if you think you can guarantee that there will never be any "holes". Only treat arrays as "lists" if you're certain, and the savings in complexity is significant enough for it to be justified.

=== Loading values into an array ===
Assigning one element at a time is simple, and portable:

{{{#!highlight bash
# 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 supports only the `arrName=(args...)` syntax. ksh88 supports only the `set -A arrName -- args...` syntax. ksh93, mksh, and zsh support both. There are subtle differences in both methods between all of these shells if you look closely.

{{{#!highlight bash
# Bash, ksh93, mksh, zsh
array=(zero one two three four)
}}}

{{{#!highlight bash
# ksh88/93, mksh, zsh
set -A array -- zero one two three four
}}}

When initializing in this way, the first index will be 0 unless a different index is specified.

With compound assignment, the space between the parentheses is evaluated in the same way as the arguments to a command, including [[glob|pathname expansion]] and WordSplitting. Any type of expansion or substitution may be used. All the usual [[Quotes|quoting]] rules apply within.

{{{#!highlight bash
# Bash/ksh93
oggs=(*.ogg)
}}}

With ksh88-style assignment using `set`, the arguments are just ordinary arguments to a command.

{{{#!highlight bash
# Korn
set -A oggs -- *.ogg
}}}

{{{#!highlight bash
# Bash (brace expansion requires 3.0 or higher)
homeDirs=(~{,root}) # brace expansion occurs in a different order in ksh, so this is bash-only.
letters=({a..z})    # Not all shells with sequence-expansion can use letters.
}}}

{{{#!highlight bash
# Korn
set -A args -- "$@"
}}}

==== Loading lines from a file or stream ====
In bash 4, the `mapfile` command (also known as `readarray`) accomplishes this:

{{{#!highlight bash
# Bash 4
mapfile -t lines <myfile

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

See ProcessSubstitution and [[BashFAQ/024|FAQ #24]] for more details on the `<(...)` syntax.

`mapfile` handles blank lines by inserting them as empty array elements, and (with `-t`) also silently appends a missing final newline if the input stream lacks one. These can be problematic when reading data in other ways (see the next section). `mapfile` in bash 4.0 through 4.3 does have one serious drawback: it can ''only'' handle newlines as line terminators.  Bash 4.4 adds the `-d` option to supply a different line delimiter.

When mapfile isn't available, we have to work '''very hard''' to try to duplicate it.  There are a great number of ways to ''almost'' get it right, but many of them fail in subtle ways.

The following examples will duplicate most of `mapfile`'s basic functionality in older shells.  '''You can skip all of these alternative examples if you have bash 4.'''

{{{#!highlight bash
# Alternative: Bash 3.1, Ksh93, mksh
unset -v lines
while IFS= read -r; do
    lines+=("$REPLY")
done <file
[[ $REPLY ]] && lines+=("$REPLY")
}}}

The `+=` operator, when used together with parentheses, appends the element to one greater than the current highest numbered index in the array.

{{{#!highlight bash
# Alternative: ksh88
# Ksh88 doesn't support pre/post increment/decrement. mksh and others do.
i=0
unset -v lines
while IFS= read -r; do
    lines[i+=1,$i]=$REPLY     # Mimics lines[i++]=$REPLY
done <file
[[ $REPLY ]] && lines[i]=$REPLY
}}}

The square brackets create a [[ArithmeticExpression|math context]]. The result of the expression is the index used for assignment.

===== Handling newlines (or lack thereof) at the end of a file =====
`read` returns false when it reads the last line of a file. This presents a problem: if the file contains a trailing newline, then `read` will be false when reading/assigning that final line, otherwise, it will be false when reading/assigning the last line of data. Without a special check for these cases, no matter what logic is used, you will always end up either with an extra blank element in the resulting array, or a missing final element.

To be clear - text files ''should'' contain a newline as the last character in the file. Newlines are added to the ends of files by most text editors, and also by [[HereDocument|Here documents]] and [[HereStromg|Here strings]]. Most of the time, this is only an issue when reading output from pipes or process substitutions, or from "broken" text files created with broken or misconfigured tools. Let's look at some examples.

This approach reads the elements one by one, using a loop.

{{{#!highlight bash
# Doesn't work correctly!
unset -v arr i
while IFS= read -r 'arr[i++]'; do
    :
done < <(printf '%s\n' {a..d})
}}}

Unfortunately, if the file or input stream contains a trailing newline, a blank element is added at the end of the array, because the `read -r arr[i++]` is executed one extra time after the last line containing text before returning false.

{{{#!highlight bash
# Still doesn't work correctly!
unset -v arr i
while read -r; do
    arr[i++]=$REPLY
done < <(printf %s {a..c}$'\n' d)
}}}

The square brackets create a [[ArithmeticExpression|math context]]. Inside them, `i++` works as a C programmer would expect (in all but ksh88).

This approach fails in the reverse case - it correctly handles blank lines and inputs terminated with a newline, but fails to record the last line of input. If the file or stream is missing its final newline.  So we need to handle that case specially:

{{{#!highlight bash
# Alternative: Bash, ksh93, mksh
unset -v arr i
while IFS= read -r; do
    arr[i++]=$REPLY
done <file
[[ $REPLY ]] && arr[i++]=$REPLY # Append unterminated data line, if there was one.
}}}

This is very close to the "final solution" we gave earlier -- handling both blank lines inside the file, and an unterminated final line. The null [[IFS]] is used to prevent `read` from stripping possible whitespace from the beginning and end of lines, in the event you wish to preserve them.

Another workaround is to remove the empty element after the loop:

{{{#!highlight bash
# Alternative: Bash
unset -v arr i
while IFS= read -r 'arr[i++]'; do
    :
done <file

# Remove trailing empty element, if any.
[[ ${arr[i-1]} ]] || unset -v 'arr[--i]'
}}}

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 [[glob|globs]].  This is also true for other non-keyword builtins that take a subscripted variable name, such as `let` and `unset`.

===== Other methods =====
Sometimes stripping blank lines actually is desirable, or you may know that the input will always be newline delimited, such as input generated internally by your script. It is possible in some shells to use the `-d` flag to set `read`'s line delimiter to null, then abuse the `-a` or `-A` (depending on the shell) flag normally used for reading the fields of a line into an array for reading lines. Effectively, the entire input is treated as a single line, and the fields are newline-delimited.

{{{#!highlight bash
# Bash 4
IFS=$'\n' read -rd '' -a lines <file
}}}

{{{#!highlight bash
# mksh, zsh
IFS=$'\n' read -rd '' -A lines <file
}}}

===== Don't read lines with for! =====
'''[[DontReadLinesWithFor|Never read lines using for..in loops]]!''' Relying on [[IFS]] WordSplitting causes issues if you have repeated whitespace delimiters, because they will be consolidated. It is not possible to preserve blank lines by having them stored as empty array elements this way. Even worse, special globbing characters will be expanded without going to lengths to disable and then re-enable it. Just never use this approach - it is problematic, the workarounds are all ugly, and not all problems are solvable.

==== Reading NUL-delimited streams ====
If you are trying to deal with records that might have embedded newlines, you will be using an alternative delimiter such as the NUL character ( \0 ) to separate the records.  In bash 4.4, you can simply use `mapfile -t -d ''`:

{{{#!highlight bash
# Bash 4.4
mapfile -t -d '' files < <(find . -name '*.ugly' -print0)
}}}

Otherwise, you'll need to use the `-d` argument to `read` inside a loop:

{{{#!highlight bash
# Bash
while read -rd ''; do
    arr[i++]=$REPLY
done < <(find . -name '*.ugly' -print0)

# or (bash 3.1 and up)
while read -rd ''; do
    arr+=("$REPLY")
done < <(find . -name '*.ugly' -print0)
}}}

`read -d ''` tells Bash to keep reading until a NUL byte instead of until a newline. This isn't certain to work in all shells with a `-d` feature.

If you choose to give a variable name to `read` instead of using `REPLY` then also be sure to set `IFS=` for the `read` command, to avoid trimming leading/trailing IFS whitespace.

==== Appending to an existing array ====
As previously mentioned, arrays are ''sparse'' - that is, numerically adjacent indexes are not guaranteed to be occupied by a value. This confuses what it means to "append" to an existing array. There are several approaches.

If you've been keeping track of the highest-numbered index with a variable (for example, as a side-effect of populating an array in a loop), and can guarantee it's correct, you can just use it and continue to ensure it remains in-sync.

{{{#!highlight bash
# Bash/ksh93
arr[++i]="new item"
}}}

If you don't want to keep an index variable, but happen to know that your array is ''not sparse'', then you can use the number of elements to calculate the offset (not recommended):

{{{#!highlight bash
# 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 don't mind re-indexing the entire array (very inefficient), then you can use:

{{{#!highlight bash
# 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:

{{{#!highlight bash
# Bash 3.1, ksh93, mksh, zsh
arr+=(item 'another item')
}}}

NOTE: the parentheses are required, just as when assigning to an array. Otherwise you will end up appending to `${arr[0]}` which `$arr` is a synonym for. If your shell supports this type of appending, it is the preferred method.

For examples of using arrays to hold complex shell commands, see [[BashFAQ/050|FAQ #50]] and [[BashFAQ/040|FAQ #40]].

=== Retrieving values from an array ===
`${#arr[@]}` or `${#arr[*]}` expand to the number of elements in an array:

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

Single elements are retrieved by index:

{{{#!highlight bash
echo "${foo[0]} - ${bar[j+1]}"
}}}

The square brackets are a [[ArithmeticExpression|math context]]. Within an arithmetic context, variables, including arrays, can be referenced by name. For example, in the expansion:

{{{#!highlight bash
${arr[x[3+arr[2]]]}
}}}

`arr`'s index will be the value from the array `x` whose index is 3 plus the value of `arr[2]`.

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,

{{{#!highlight bash
# 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:

{{{#!highlight bash
# 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.

{{{#!highlight bash
# Bash
arr=(x y z)
IFS=/; echo "${arr[*]}"; unset -v 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:

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

Or using array slicing, described in the next section.

{{{#!highlight bash
# Bash/ksh
typeset -a a=([0]=x [5]=y [10]=z)
printf '%s<=>' "${a[@]::${#a[@]}-1}"
printf '%s\n' "${a[@]:(-1)}"
}}}

This also shows how sparse arrays can be assigned multiple elements at once. Note using the `arr=([key]=value ...)` notation differs between shells. In ksh93, this syntax gives you an associative array by default unless you specify otherwise, and using it requires that every value be explicitly given an index, unlike bash, where omitted indexes begin at the previous index. This example was written in a way that's compatible between the two.

BASH 3.0 added the ability to retrieve the list of index values in an array:

{{{#!highlight bash
# Bash 3.0 or higher
arr=(0 1 2 3) arr[42]='what was the question?'
unset -v 'arr[2]'
echo "${!arr[@]}"
# prints 0 1 3 42
}}}

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

{{{#!highlight bash
# Bash 3.0 or higher
unset -v 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 sparse, 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
}}}

==== Retrieving with modifications ====
Bash's [[BashFAQ/073|Parameter Expansions]] may be performed on array elements ''en masse'':

{{{#!highlight bash
# 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 sub-lists of elements from an array.  Some people call this ''slicing'':

{{{#!highlight bash
# Bash
echo "${arr[@]:1:3}"        # three elements starting at #1 (second element)
echo "${arr[@]:(-2)}"       # last two elements
}}}

The same goes for positional parameters

{{{#!highlight bash
set -- foo bar baz
echo "${@:(-1)}"            # last positional parameter baz
echo "${@:(-2):1}"          # second-to-last positional parameter bar
}}}

=== Using @ as a pseudo-array ===
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:

{{{#!highlight bash
# POSIX
set -- *.mp3
if [ -e "$1" ] || [ -L "$1" ]; then
  echo "there are $# MP3 files"
else
  echo "there are 0 MP3 files"
fi
}}}

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

(Compare to [[BashFAQ/050|FAQ #50]]'s dynamically generated commands using named arrays.)

== See Also ==
 * [[BashGuide/Arrays]]
 * [[BashSheet#Arrays|BashSheet Array reference]]
 * [[BashFAQ/006#Associative_Arrays|BashFAQ 6 - explaining associative arrays]]

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CategoryShell