Debanjan .Mukherjee Debanjan .Mukherjee - 1 month ago 4
Bash Question

compare substrings in Busybox ash

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 :

[[ $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$A* ] and [ ā€œ$Iā€ == ā€œ$N$Aā€* ] 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 ]]
echo "sale is complete"
echo "sale is declined"

All i need is -->

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

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


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.