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Differences in BASH and ASH shells regarding if command?

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# 1  
Old 09-09-2010
Differences in BASH and ASH shells regarding if command?


I now have a script that's working in a BASH environment, however one line doesn't appear to be working on an embedded device that has a busybox therefore ASH shell. I've googled but there's very little I can find regarding the ASH shell.

In BASH the following line works...

if [[ "$TARGETLIST" == *$QUERYVALUE* ]] ; then 
do blah blah blah

The line is looking for the $QUERYVALUE to appear anywhere in the list of entries in $TARGETLIST and if it does it executes blah blah blah.

However in busybox the command just doesn't work (i.e. it doesn't detect if the $QUERYVALUE is in the $TARGETLIST) and doesn't throw an error.

Any ideas?
# 2  
Old 09-09-2010
ASH is of course extremely limited, though it's amazing for what it is: Its source code is smaller than the compiled binaries of some other shells.

I thought globbing that way was a bash-ism but it seems to work in ksh too. Interesting.

I'd use string operations instead of glob-matching to test if a variable resides inside another in ASH. Replace the matching part with nothing, and if the resulting string is smaller, then it must have matched.

$ NEEDLE="ert"
$ HAYSTACK="quwertyuiop"
$ echo $T
$ if [[ "${#T}" -lt "${#HAYSTACK}" ]]
        echo "NEEDLE is in HAYSTACK"

Last edited by Corona688; 09-09-2010 at 01:59 PM.. Reason: more quotes igor!
# 3  
Old 09-09-2010
ash is (almost) a posix shell and offers no additional functionality and as such does not support [[ ]] with pattern matching (which is ksh/bash)
  *$QUERYVALUE*) blah blah blah

# 4  
Old 09-09-2010
Thanks Guys

I went Coronas way as I also wanted to incorporate the queryvariable changing and I could visualise the code better in my head.

What now happens is the queyvalue gets read from a blacklist table and then tested against the 'master' tables, if there's a match it sets a bypass flag and moves onto the next 'master' table.

The whole script is working now, I just have to do some better formatting of the output to make it easier to read.

Thanks once again.
# 5  
Old 09-13-2010
The Ash Shell on Busybox is well documented on this site. The exact version of Ash depends on the vintage of your Busybox.

BusyBox - The Swiss Army Knife of Embedded Linux

There is a list of Ash variants (including Dash) on this site.
# 6  
Old 09-13-2010

I'd read that previously but it's not that helpful as for example the 'if' command isn't covered so it didn't help me with allowed structures.

# 7  
Old 09-13-2010
This may give you a good idea of what can be used in ash:
Posix Shell & Utilities
Specifically about your question:
If Conditional Construct and test - evaluate expression ( [expression] )
Case Conditional Construct and Pattern Matching Notation

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