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| If your real question was ''How do I check whether one of my parameters was -v?'' then please see [[BashFAQ/035|FAQ #35]] instead. Otherwise, read on.... | If your real question is ''How do I check whether one of my parameters was -v?'' then see [[BashFAQ/035|FAQ #35]] instead. Otherwise, read on… |
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| First of all, let's get the terminology straight. Bash has no notion of "lists" or "sets" or any such. Bash has strings and [[BashFAQ/005|arrays]]. Strings are a "list" of '''characters''', arrays are a "list" of '''strings'''. | <<TableOfContents>> === Associative arrays === All we need to do is create an entry for each item and look it up by index. In this example, we test whether the user input `x` is a member of the set `a`: |
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| '''NOTE:''' In the general case, a string cannot possibly contain a list of other strings because there is no reliable way to tell where each substring begins and ends. | {{{ # Bash etc. |
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| Given a traditional array, the only proper way to do this is to loop over all elements in your array and check them for the element you are looking for. Say what we are looking for is in `bar` and our list is in the array `foo`: {{{ # Bash for element in "${foo[@]}"; do [[ $element = $bar ]] && echo "Found $bar." done }}} |
function get_input { [[ -t 0 ]] || return printf 'hm? ' IFS= read -r${BASH_VERSION+\e} -- "$1" } |
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| If you need to perform this several times in your script, you might want to extract the logic into a function: {{{ # Bash isIn() { local pattern="$1" element shift for element do [[ $element = $pattern ]] && return 0 done return 1 } if isIn "jacob" "${names[@]}" then echo "Jacob is on the list." fi }}} |
set -- Bigfoot UFOs Republicans typeset -A a for x; do a+=([$x]=) done |
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| Or, if you want your function to return the '''index''' at which the element was found: {{{ # Bash 3.0 or higher indexOf() { local pattern=$1 local index list shift list=("$@") for index in "${!list[@]}" do [[ ${list[index]} = $pattern ]] && { echo $index return 0 } done echo -1 return 1 } if index=$(indexOf "jacob" "${names[@]}") then echo "Jacob is the ${index}th on the list." else echo "Jacob is not on the list." fi }}} |
get_input x |
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| If your "list" is contained in a string, and for some half-witted reason you choose not to heed the warnings above, you can use the following code to search through "words" in a string. (The only real excuse for this would be that you're stuck in Bourne shell, which has no arrays.) {{{ # Bourne set -f for element in $foo; do if test x"$element" = x"$bar"; then echo "Found $bar." fi done set +f }}} |
if [[ -v a[$x] ]]; then printf '%s exists!\n' "$x" else printf '%s doesn't exist.\n' "$x" fi }}} |
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| Here, a "word" is defined as any substring that is delimited by whitespace (or more specifically, the characters currently in IFS). The `set -f` prevents [[glob]] expansion of the words in the list. Turning glob expansions back on (`set +f`) is optional. | === Indexed arrays === We can store a list of strings in an indexed array by looping over each element: |
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| If you're working in bash 4 or ksh93, you have access to associative arrays. These will allow you to restructure the problem -- instead of making a list of words that are allowed, you can make an ''associative array'' whose keys are the words you want to allow. Their values could be meaningful, or not -- depending on the nature of the problem. | {{{ # Bash |
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| {{{ # Bash 4 declare -A good for word in "goodword1" "goodword2" ...; do good["$word"]=1 done |
typeset -a haystack for x in "${haystack[@]}"; do [[ $x == "$needle" ]] && printf 'Found %q!\n' "$needle" done }}} |
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| # Check whether $foo is allowed: if ((${good[$foo]})); then ... }}} |
=== enum (ksh93) === In ksh93t or later, one may create enum types/variables/constants using the `enum` builtin. These work similarly to C enums (and the equivalent feature of other languages). These may be used to restrict which values may be assigned to a variable so as to avoid the need for an expensive test each time an array variable is set or referenced. Like types created using `typeset -T`, the result of an `enum` command is a new declaration command that can be used to instantiate objects of that type. |
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| Here's a hack that you shouldn't use, but which is presented for the sake of completeness: {{{ # Bash if [[ " $foo " = *" $bar "* ]]; then echo "Found $bar." fi }}} (The problem here is that is assumes ''space'' can be used as a delimiter between words. Your elements might contain spaces, which would break this!) |
{{{ # ksh93 |
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| That same hack, for Bourne shells: {{{ # Bourne case " $foo " in *" $bar "*) echo "Found $bar.";; esac }}} |
$ enum colors=(red green blue) $ colors foo=green $ foo=yellow ksh: foo: invalid value yellow }}} |
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| You can also use extended glob with printf to search for a word in an array. ''I haven't tested it enough, so it might break in some cases --sn18'' |
`typeset -a` can also be used in combination with an enum type to allow enum constants as subscripts. |
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| {{{ # Bash shopt -s extglob #convert array to glob printf -v glob '%q|' "${array[@]}" glob=${glob%|} [[ $word = @($glob) ]] && echo "Found $word" }}} |
{{{ # ksh93 |
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| . ''It will break when an array element contains a | character. Hence, I moved it down here with the other hacks that work in a similar fashion and have a similar limitation.'' -- GreyCat GNU's grep has a {{{\b}}} feature which allegedly matches the edges of words. Using that, one may attempt to replicate the shorter approach used above, but it is fraught with peril: {{{ # Is 'foo' one of the positional parameters? egrep '\bfoo\b' <<<"$@" >/dev/null && echo yes # This is where it fails: is '-v' one of the positional parameters? egrep '\b-v\b' <<<"$@" >/dev/null && echo yes # Unfortunately, \b sees "v" as a separate word. # Nobody knows what the hell it's doing with the "-". # Is "someword" in the array 'array'? egrep '\bsomeword\b' <<<"${array[@]}" # Obviously, you can't use this if someword is '-v'! }}} Since this "feature" of GNU grep is both non-portable and poorly defined, we recommend '''not''' using it. It is simply mentioned here for the sake of completeness. == Bulk comparison == This method tries to compare the desired string to the entire contents of the array. It can potentially be very efficient, but it depends on a delimiter that must not be in the sought value or the array. Here we use $'\a', the BEL character, because it's extremely uncommon. {{{ # usage: if has "element" list of words; then ...; fi has() { local IFS=$'\a' t="$1" shift [[ $'\a'"$*"$'\a' == *$'\a'$t$'\a'* ]] } }}} |
$ typeset -a '[colors]' bar $ bar[blue]=test1 $ typeset -p bar typeset -a '[colors]' bar=([blue]=test) $ bar[orange]=test ksh: colors: invalid value orange }}} |
I want to check to see whether a word is in a list (or an element is a member of a set).
If your real question is How do I check whether one of my parameters was -v? then see FAQ #35 instead. Otherwise, read on…
Contents
Associative arrays
All we need to do is create an entry for each item and look it up by index. In this example, we test whether the user input x is a member of the set a:
# Bash etc.
function get_input {
[[ -t 0 ]] || return
printf 'hm? '
IFS= read -r${BASH_VERSION+\e} -- "$1"
}
set -- Bigfoot UFOs Republicans
typeset -A a
for x; do
a+=([$x]=)
done
get_input x
if [[ -v a[$x] ]]; then
printf '%s exists!\n' "$x"
else
printf '%s doesn't exist.\n' "$x"
fi
Indexed arrays
We can store a list of strings in an indexed array by looping over each element:
# Bash
typeset -a haystack
for x in "${haystack[@]}"; do
[[ $x == "$needle" ]] && printf 'Found %q!\n' "$needle"
done
enum (ksh93)
In ksh93t or later, one may create enum types/variables/constants using the enum builtin. These work similarly to C enums (and the equivalent feature of other languages). These may be used to restrict which values may be assigned to a variable so as to avoid the need for an expensive test each time an array variable is set or referenced. Like types created using typeset -T, the result of an enum command is a new declaration command that can be used to instantiate objects of that type.
# ksh93 $ enum colors=(red green blue) $ colors foo=green $ foo=yellow ksh: foo: invalid value yellow
typeset -a can also be used in combination with an enum type to allow enum constants as subscripts.
# ksh93 $ typeset -a '[colors]' bar $ bar[blue]=test1 $ typeset -p bar typeset -a '[colors]' bar=([blue]=test) $ bar[orange]=test ksh: colors: invalid value orange