How to remove an item from array in bash?

rootbox · 2012-09-06 09:59:27

Hi

i tried serveral methods to remove an specific item from an array:

for host in ${HOSTNAMES[@]}
do
if [ "${host}" != $HOSTNAME ] ; then
ping $host 1 &> /dev/null
if test "$?" -eq "0" ; then
echo "ok"
else
#here i want to remove $host from the array HOSTNAMES
fi
fi
done

anyone have an idea, how i can remove the host?

Edit: The Device i want to run this script is an Unix System (Sun)

Last edited by rootbox (2012-09-06 10:01:41)

Trilby · 2012-09-06 10:11:35

I rarely use bash arrays - so consider this as one option that will likely be superseeded by better recommendations. I'd copy the array into a new temporary array which could then be moved back to the HOSTNAMES array after the loop. In other words, instead of trying to conditionally excise elements, conditionally append the items that pass the test to a new array.

skanky · 2012-09-06 10:17:30

You'll need the index to clear:

unset HOSTNAMES[index]

Edit, incidentally you can skip empty items in the array by calling:

for host in {!HOSTNMES[@]}...etc...

Last edited by skanky (2012-09-06 10:19:05)

rootbox · 2012-09-06 10:47:14

So i tried:

i=0
ii=0
for host in ${HOSTNAMES[@]}
do
let i=i+1
let ii=i-1
echo $ii
if [ "${host}" != $HOSTNAME ] ; then
ping $host 1 &> /dev/null
if test "$?" -eq "0" ; then
echo "ok"
else
unset HOSTNAMES[$i]
echo ${HOSTNAMES[@]}
fi
fi
done

but for any reasons it says:
./script: line 103: unset: HOSTNAMES: not an array variable

rootbox · 2012-09-06 11:00:31

i think this methode with the counter is crap

when i remove one item from the array the counter doesn't match anymore

Trilby · 2012-09-06 11:49:44

When you remove one from the array, you should decrement the counter. I have no idea why you have two counters. Alternately, only increment the counter in the conditional code for when you dont remove and item.

So "if condition then incremement counter, else remove item at current position"

Last edited by Trilby (2012-09-06 11:51:32)

skanky · 2012-09-06 12:49:44

Do you populate the array? If so you could make it an associative array, and reference it thus:

HOSTNAMES[host]=host

That way you can remove it thus:

unset HOSTNAMES[$host]

Bit of a hack, but should work.

Otherwise you could just set it to an empty string and ignore empty strings in your processing. Not ideal.

Otherwise there are tricks with indexes and array sizes that are indicated in the Bash manual. I'd recommend reading the Arrays section of it.

rootbox · 2012-09-06 13:10:22

Trilby wrote:

When you remove one from the array, you should decrement the counter. I have no idea why you have two counters. Alternately, only increment the counter in the conditional code for when you dont remove and item.
So "if condition then incremement counter, else remove item at current position"

That works

I have to counter because $i counts one up so it starts with 1 but the array start with 0 so -1 in $ii

rockin turtle · 2012-09-06 19:10:36

A problem with unset is that it will remove an item, but will not re-index the array.

#!/bin/bash

# create an array
xyz=(a b c d e)
echo ${xyz[@]}
orig_length=${#xyz[@]}

# remove an item
unset xyz[2]

echo ${xyz[@]}
echo

for (( i=0; i < orig_length; i++ )); do
	echo "$i: ${xyz[i]}"
done

If you are using bash as your shell, you can do the following to remove the desired host from an array. (This may not work in other shells.)

#!/bin/bash

HOSTNAMES=(a b c d e f c)
HOSTNAME=c

echo "${HOSTNAMES[@]}"
HOSTNAMES=($(for h in ${HOSTNAMES[@]}; do [ "$h" != "$HOSTNAME" ] && echo $h; done ))
echo "${HOSTNAMES[@]}"

#for host in "${HOSTNAMES[@]}"; do
#	ping "$host" 1 &> /dev/null && echo "${host}... ok"
#done

skanky · 2012-09-06 19:35:08

rockin turtle wrote:

A problem with unset is that it will remove an item, but will not re-index the array.

That doesn't necessarily matter if you reference using

${!xyz[@]}

which will ignore any unset items.

That's just for future reference though.

rockin turtle · 2012-09-06 21:08:51

skanky is correct, this is probably your best approach

for i in "${!HOSTNAMES[@]}"; do
	host="${HOSTNAMES[i]}"
	if [ "$host" != "$HOSTNAME" ]
		ping "$host" 1 &> /dev/null && echo "${host}... ok"
	else
		unset HOSTNAMES[i]
	fi
done

Edit: Oops, I see I got the logic from rootbox's original script incorrect.

Perhaps this will work.

for i in ${!HOSTNAMES[@]}; do
	host="${HOSTNAMES[i]}"
	[ "$host" == "$HOSTNAME" ] && continue
	ping $host 1 &> /dev/null || unset HOSTNAME[i]
done

Last edited by rockin turtle (2012-09-06 21:38:52)

skanky · 2012-09-06 21:16:23

Yes sorry, I had forgotten that it expanded the array *keys* which makes it much more useful, and had posted/implied it returned the values.
It was checking rockin turtle's #11 solution that reminded me of the error.
My mistake and well caught.

Arch Linux

#1 2012-09-06 09:59:27

How to remove an item from array in bash?

#2 2012-09-06 10:11:35

Re: How to remove an item from array in bash?

#3 2012-09-06 10:17:30

Re: How to remove an item from array in bash?

#4 2012-09-06 10:47:14

Re: How to remove an item from array in bash?

#5 2012-09-06 11:00:31

Re: How to remove an item from array in bash?

#6 2012-09-06 11:49:44

Re: How to remove an item from array in bash?

#7 2012-09-06 12:49:44

Re: How to remove an item from array in bash?

#8 2012-09-06 13:10:22

Re: How to remove an item from array in bash?

#9 2012-09-06 19:10:36

Re: How to remove an item from array in bash?

#10 2012-09-06 19:35:08

Re: How to remove an item from array in bash?

#11 2012-09-06 21:08:51

Re: How to remove an item from array in bash?

#12 2012-09-06 21:16:23

Re: How to remove an item from array in bash?

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