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.

Answer 2

A bit unrelated to the issue, but I ran across this post when attempting to compare date strings and thought it would be useful for anyone with a similar issue. To use greater or less than on strings in ash you will need a forward slash before your comparison character:

str2="2020-1-1"
str3="2020-1-2"
# greater or less than
if [ "$str2" \> "$str3" ]; then
    echo "$str2 is greater then $str3"
elif [ "$str2" \< "$str3" ]; then
    echo "$str2 is less then $str3"
fi

Answer 3

Since I ended up here while searching for how to do the exact same thing, with Busybox compiled with CONFIG_ASH_BASH_COMPAT=y I ended up using the equivalent of this:

if [[ "${I//$N$A}" != "$I" ]
then
  echo "sale is complete"  
else  
  echo "sale is declined"  
fi   

ie, using the pattern replacement operator to try to remove the substring I am looking for, and if it modifies the string it did contain the substring.