[SOLVED] #!/bin/sh vs #!/bin/bash -- script executes differently; each way = different bugs

GrapefruiTgirl · 12-07-2009, 02:55 PM

Quote:

Originally Posted by tuxdev

For doing stuff that's mathematical, it's actually better to use (()) arithmetic test construct

One thing I have learned recently, which can be seen in another couple of threads somewhere here, is using `bc` to do the math

-- not only does it not gag when you input "Strings" of numbers, but it does floating point too. Very handy for anything beyond the most mundane basic math, and simple do use:

shell$ result="$(echo "2.54 * 3.6" | bc -l)"

Anyhow, maybe I'm drifting off topic here

Sasha

tuxdev · 12-07-2009, 03:08 PM

Yeah, with a decimal point, you're pretty much stuck with bc.

The nice thing about (( )) is that it doesn't gag either. When all you area doing is the most mundane basic math, it's much nicer to do it directly in the shell rather than fork off a process.

H_TeXMeX_H · 12-08-2009, 01:24 PM

Quote:

Originally Posted by tuxdev

Yeah, with a decimal point, you're pretty much stuck with bc.

The nice thing about (( )) is that it doesn't gag either. When all you area doing is the most mundane basic math, it's much nicer to do it directly in the shell rather than fork off a process.

or awk ...

GrapefruiTgirl · 12-08-2009, 01:49 PM

For those interested:

I played around with different shebangs at the beginning of my script; using #!/bin/sh works, as does #!/bin/bash --posix, but using just #!/bin/bash causes weird errors here and there.

So, the good news is that (at least in theory, and in practice during my testing) my script is POSIX compliant, which was my hope.

Sasha

GrapefruiTgirl · 12-16-2009, 02:52 PM

Since this thread is also related to all this stuff:

http://www.linuxquestions.org/questi...script-775058/
http://www.linuxquestions.org/questi...rings.-775622/

.. and the above stuff is [solved], I consider this issue solved too. The problem was in part that bash --posix is not actually posix compliant, and code that will run under that, will NOT necessarily run in a real POSIX shell.

Also, FWIW, AFAICT-IIRC using [[..]] is also not POSIX compatible, so I've opted to keep all of my [..] tests as they are, at least for the project I'm currently working on.

Thanks for everyones interest & input here

Sasha

tuxdev · 12-16-2009, 05:07 PM

You've discovered the hard way a lot of what's said here, but it might be an interesting read nonetheless:
http://mywiki.wooledge.org/BashGuide...hooseYourShell

MBybee · 12-16-2009, 05:30 PM

Quote:

Originally Posted by GrapefruiTgirl

OK, cool -- thanks you guys for the tips about the double brackets; I've read of it MANY times but never found that I actually needed to do it, although I have encountered the operator errors within the [ .. ] constructs, and generally got around it by switching operator type, such as from | to -o or from != to -ne. I always wrote this off as bash telling me that I was using the wrong operator for the type of variable, i.e. trying to compare $string to $integer ]. I had pretty much given up on < > and switched to -lt -gt instead, but this double-bracket thing will have me re-examining this stuff.

Thanks!!
Sasha

With single [ you should always stick to the POSIX compliant operators, as pointed out already. If you're going pure bash you might as well take advantage of [[ ]] and such

This seems complex enough that you may want to look into awk or Perl (as I see was mentioned earlier), just because shell script can get 'interesting' when you go beyond a certain level.