18
9
Is there a nice way of checking if an array has an element in bash (better than looping through)?
Alternatively, is there another way to check if a number or string equals any of a set of predefined constants?
18
9
Is there a nice way of checking if an array has an element in bash (better than looping through)?
Alternatively, is there another way to check if a number or string equals any of a set of predefined constants?
25
In Bash 4, you can use associative arrays:
# set up array of constants
declare -A array
for constant in foo bar baz
do
array[$constant]=1
done
# test for existence
test1="bar"
test2="xyzzy"
if [[ ${array[$test1]} ]]; then echo "Exists"; fi # Exists
if [[ ${array[$test2]} ]]; then echo "Exists"; fi # doesn't
To set up the array initially you could also do direct assignments:
array[foo]=1
array[bar]=1
# etc.
or this way:
array=([foo]=1 [bar]=1 [baz]=1)
10
It's an old question, but I think what is the simplest solution has not appeared yet: test ${array[key]+_}
. Example:
declare -A xs=([a]=1 [b]="")
test ${xs[a]+_} && echo "a is set"
test ${xs[b]+_} && echo "b is set"
test ${xs[c]+_} && echo "c is set"
Outputs:
a is set
b is set
To see how this work check this.
2The info manual recommneds you to use env
to avoid ambiguities in aliases, progs and other functions that may have adopted the name "test". As above env test ${xs[a]+_} && echo "a is set"
. You can also get this functionality using double-brackets, the same trick then checking for null: [[ ! -z "${xs[b]+_}" ]] && echo "b is set"
– A.Danischewski – 2016-02-01T13:44:12.527
Also you can use the even simpler [[ ${xs[b]+set} ]]
– Arne L. – 2019-08-01T10:18:12.317
5
There is a way to test if an element of an associative array exists (not set), this is different from empty:
isNotSet() {
if [[ ! ${!1} && ${!1-_} ]]
then
return 1
fi
}
Then use it:
declare -A assoc
KEY="key"
isNotSet assoc[${KEY}]
if [ $? -ne 0 ]
then
echo "${KEY} is not set."
fi
just a note: declare -A doesn't work on bash 3.2.39 (debian lenny), but it works on bash 4.1.5 (debian squeeze) – Paweł Polewicz – 2011-12-14T16:05:27.823
Associative arrays were introduced in Bash 4. – Diego F. Durán – 2011-12-20T15:23:36.260
1note that if ! some_check then return 1
= some_check
. So: isNotSet() { [[ ... ]] }
. Check my solution below, you can do it in a simple check. – tokland – 2012-05-25T22:16:09.890
3
You can see if an entry is present by piping the contents of the array to grep.
printf "%s\n" "${mydata[@]}" | grep "^${val}$"
You can also get the index of an entry with grep -n, which returns the line number of a match (remember to subtract 1 to get zero-based index) This will be reasonably quick except for very large arrays.
# given the following data
mydata=(a b c "hello world")
for val in a c hello "hello world"
do
# get line # of 1st matching entry
ix=$( printf "%s\n" "${mydata[@]}" | grep -n -m 1 "^${val}$" | cut -d ":" -f1 )
if [[ -z $ix ]]
then
echo $val missing
else
# subtract 1. Bash arrays are zero-based, but grep -n returns 1 for 1st line, not 0
echo $val found at $(( ix-1 ))
fi
done
a found at 0
c found at 2
hello missing
hello world found at 3
explanation:
$( ... )
is the same as using backticks to capture output of a command into a variable printf
outputs mydata one element per line @
instead of *.
this avoids splitting "hello world" into 2 lines) grep
searches for exact string: ^
and $
match beginning and end of line grep -n
returns line #, in form of 4:hello world grep -m 1
finds first match onlycut
extracts just the line number You can of course fold the subtraction into the command. But then test for -1 for missing:
ix=$(( $( printf "%s\n" "${mydata[@]}" | grep -n -m 1 "^${val}$" | cut -d ":" -f1 ) - 1 ))
if [[ $ix == -1 ]]; then echo missing; else ... fi
$(( ... ))
does integer arithmetic1
#!/bin/bash
function in_array {
ARRAY=$2
for e in ${ARRAY[*]}
do
if [[ "$e" == "$1" ]]
then
return 0
fi
done
return 1
}
my_array=(Drupal Wordpress Joomla)
if in_array "Drupal" "${my_array[*]}"
then
echo "Found"
else
echo "Not found"
fi
1Can you elaborate on why you are suggesting this approach? OP asked if there is a way to do it without looping through the array, which is what you are doing in in_array
. Cheers – bertieb – 2018-02-27T11:28:16.313
Well, at least that loop is capsuled in a function, which might be good enough for many cases (with small data sets), and doesn't require bash 4+. Probably ${ARRAY[@]}
should be used. – Tobias – 2018-09-26T08:49:43.297
1
I don't think you can do it properly without looping unless you have very limited data in the array.
Here is one simple variant, this would correctly say that "Super User"
exists in the array. But it would also say that "uper Use"
is in the array.
MyArray=('Super User' 'Stack Overflow' 'Server Fault' 'Jeff' );
FINDME="Super User"
FOUND=`echo ${MyArray[*]} | grep "$FINDME"`
if [ "${FOUND}" != "" ]; then
echo Array contains: $FINDME
else
echo $FINDME not found
fi
#
# If you where to add anchors < and > to the data it could work
# This would find "Super User" but not "uper Use"
#
MyArray2=('<Super User>' '<Stack Overflow>' '<Server Fault>' '<Jeff>' );
FOUND=`echo ${MyArray2[*]} | grep "<$FINDME>"`
if [ "${FOUND}" != "" ]; then
echo Array contains: $FINDME
else
echo $FINDME not found
fi
The problem is that there is no easy way to add the anchors (that I can think of) besides looping through the array. Unless you can add them before you put them in the array...
It is a nice solution when the constants are alphanumeric, though (with grep "\b$FINDME\b"
). Probably could work with non-alphanumeric constants that have no spaces, with "(^| )$FINDME(\$| )"
(or something like that... I have never been able to learn what flavor of regexp grep uses.) – Tgr – 2010-11-30T11:10:38.130
Actually, the [[]] test doesn't work in the case the value is empty. E.g., "array['test']=''". In this case, the key 'test' exists, and you can see it listed with ${!array[@]}, but "[[ ${array['test']} ]]; echo $?" echoes 1, not 0. – haridsv – 2011-06-06T01:59:00.823
1
${array[$test1]}
is simple but has a problem: it won't work if you useset -u
in your scripts (which is recommended), as you'd get "unbound variable". – tokland – 2012-05-25T21:53:12.167@tokland: Who recommends it? I certainly don't. – Paused until further notice. – 2012-05-25T22:00:54.017
@DennisWilliamson: Ok, some people recommend it, but I think it would be nice to have a solution that works regardless the value of these flags. – tokland – 2012-05-25T22:03:34.210
Some posts about
– tokland – 2012-05-25T22:15:03.353set -u
: http://www.davidpashley.com/articles/writing-robust-shell-scripts.html, http://blog.hashbang0.com/2010/05/18/robust-bash-scripts-part-one/, http://www.openews.net/2012/bash-script-to-write-robust/.