compare substrings in Busybox ash

Question

This is my first time on stackoverflow .I am facing an issue currently ,and sharing the details here.

I am currently building a POS automation script . The POS terminal have Shell Busybox ash . Thats why am not able to use the basic commands ,as those are not behaving same . Below is the query :

Suppose [[ $I == $N$A ]] - this comparison is for exact match ,where $I is the bigger string and $N$A contains the substring of $I. i have used [ -z ${I##$N$A* ] and [ “$I” == “$N$A”* ] syntax to compare the substring , but it fails and not behaving like it should.

Please guide if any one have any suggestion on this . Please let me know if there is any online console for busybox ash where i can test some scripts .

Example Added -27-08-16

suppose - the script deriving the value $I = "Credit.saleApproved" and i am passing the value for $N= "Credit" and $A= ".sale"
So basically echo $N$A is a substring of echo $I I am writing this pseudo logic for ur better understanding

If  [[ $I == $N$A ]]  
then  
echo "sale is complete"  
else  
echo "sale is declined"  
fi   

All i need is -->

1 . input : $I = Credit.saleApproved  
          $N$A = Credit.sale  
    Output :sale is complete  

2.input : $I = Credit.sApproved  
          $N$A = Credit.sale  
    Output :sale is Declined  

Answer 1

The Bourne Again SHell supports some comparisons that are not supported by other shells, such as Busybox ash. Some common pitfalls are enlisted here

Specifically comparison with [[ ... ]] are only supported by bash, as well as using a wildcard (*) in comparisons.

If you would like to match using ash, you could try these:

[ "$I" == "$N$A" ] # Match exactly
[ "$I" != "${I#$N$A}" ] # Starts with
[ "$I" != "${I%$N$A}" ] # Ends with

To check whether a string contains some other string, I can think of no easy way to do that with shell expressions, ash does not support string substitution like ${I/$N$A}. There are multiple tools to choose from, for example grep and sed.

Using grep you could do:

if echo $I|grep "$N$A" - > /dev/null; then ...

Using sed you could do:

[ -z $(echo "$I"|sed "/$N$A/d") ] # Contains

But there are many ways to achieve this.