Q. How do I define array in a bash shell script? How do I find out bash array length (number of elements) while running a script using for shell loop?
A. Bash provides one-dimensional array variables. Any variable may be used as an array; the declare builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously.
How do I define bash array?
Array can be defined using following syntax:
ArrayName=("element 1" "element 2" "element 3")
Define array called distro with 3 elements, enter:
distro=("redhat" "debian" "gentoo") |
How do I reference any element in bash array?
Any element of an array may be referenced using following syntax:
${ArrayName[subscript]} |
To print redhat i.e first element enter:
echo ${distro[0]} echo ${distro[2]} # will print gentoo |
How do I find out bash shell array length?
You can easily find out bash shell array length using following syntax:
${#ArrayName[@]} |
To print distro array length enter:
echo ${#distro[@]} |
Sample output:
3
If subscript is @ or *, the word expands to all members of name. By prefixing # to variable you will find length of an array (i.e number of elements).
Putting it all together
A sample shell script to print array called NAMESERVERS:
#!/bin/bash # define array # name server names FQDN NAMESERVERS=("ns1.nixcraft.net." "ns2.nixcraft.net." "ns3.nixcraft.net.") # get length of an array tLen=${#NAMESERVERS[@]} # use for loop read all nameservers for (( i=0; i<${tLen}; i++ )); do echo ${NAMESERVERS[$i]} done |
Sample output:
ns1.nixcraft.net. ns2.nixcraft.net. ns3.nixcraft.net.
17 comment
Nice one ;)
Excellent info.
how to clean the array? ;d
Here’s a quick flush:
NUM=${#a[@]}; for ((i=0;i<${NUM};i++)); do a[${i}]=''; done
And a for a quick scan, you can do the following either before or after:
COUNT=0; while [ ${COUNT} -lt ${#a[@]} ]; do echo ${a[${COUNT}]}; COUNT=$((COUNT+1)); done
Hi!
I think that this example is bad.
I would do this without a block .
#!/bin/bash
NAMESERVERS=”ns1.nixcraft.net. ns2.nixcraft.net. ns3.nixcraft.net.”
for i in $NAMESERVERS; do
echo “$i”
done
That only works if you have no spaces.
The way it’s done in the post only works when using bash. This is a more POSIX-compatible way of making it work. You can also change the element-separator character to something different than the space, using the variabe IFS.
Problems,
a[2]=222
echo ${#a[@]}
1
Thus this is the lenght of the array, not the index of the last element.
Try instead:
for i in “${a[@]}”; do echo $i; done
Some further ideas may be found at
http://tldp.org/LDP/abs/html/arrays.html
http://www.linuxjournal.com/content/bash-arrays
Best Regards,
Raymond
Well yes, ${#a[@]} pretty much spits out the the number of elements in the array, like the document here said… notice that it’s plainly used as tlen which is the length of the array?
you could do it a million different ways…
NUM=${a[@]}
for ((i=0;i<${NUM};i++)); do echo ${a[${NUM}]}; done
or:
COUNT=0; while [ ${COUNT} -lt ${#a[@]} ]; do echo ${a[${COUNT}]}; COUNT=$((COUNT+1)); done
We could go on and on, since there's more than one way to skin a cat.
Good points on handling a possibly sparse array, Raymond, but the example is pretty useful if you load the array yourself. Also note that you don’t need to use a variable to store the array length:
for (( i = 0 ; i < ${#my_array[@]} ; i++ )) do; echo "${my_array[i]}"; done
Note the implementations of BASH I am using don't require "$i" as the index of the array, they work fine with just "i". This only seems to work with single character length index variable names though, like "i", not with, for example, "index". YMMV.
Regards,
Stuart
1.Write a Linux shell Script to count the Number of user accounts both normal and special or privileged user user accounts on the system.
2.Write an interactive Linux shell script to test whether a file is (a). Read Permission set. (b). Execution permission set (c). Is a non empty file (d). A regular file (e). Ia a directory.
lol @ all the bikeshedding syntax scholars here. Ok, your voice has been heard, move along idiots.
It’s called sharing knowledge. If you don’t like it, move along.
Dan I know this is an older thread but I found this and your information was very helpful to me. Unfortunately there are not enough people in the world like you and more like Ryan.
Thanks again!
Hi, Sorry i know this is an old thread!
I need a help!
for i in `seq 1 ${#DEV[@]}` ; do echo "DEV- ${DEV[$i]} ${DEV_DS[$i]} " >> $REPORT1${#DEV[@]} –> this represents the length of the array
If I want to go to the second last element of the array , what can I write in place of ${#DEV[@]} ?
If ${#DEV[@]} is the last element then just decrement it to get the second last element.
Old thread; yep i’ve used below;
$(( ${#arry[@]}-2 ))