check variable content size in sh script
Devin.Teske at fisglobal.com
Sun May 19 00:06:50 UTC 2013
On May 18, 2013, at 4:54 PM, Quartz wrote:
>> #foo works with sh
> Is it actually part of the official spec though is what I'm wondering, or is it a case of other shells not rejecting 'advanced' statements when running in emulation.
Shells don't have an "emulation mode". The shell supports what it supports, and no shell that I've ever used had an "emulation mode" to act like another shell.
Maybe you're referring to as "emulation" is actually the invocation line of the shell script.
Make no mistake… when you change the invocation (first) line of a shell script from:
You are not instructing a shell to "emulate" anything, you are actually using a different shell.
sh != tcsh != bash != ash != dash != zsh
Your script will use the shell that is written in the innovation line and the features you get are respective to which shell you choose.
>> At least FreeBSD's implementation of sh (which is ash, I think)
>> supports the # functionality.
> The reason I say all this is that my copy of tcsh (on this not-freebsd machine) *doesn't* work with this when in sh emulation.
Get the idea that "csh" or "tcsh" are *anything* like sh out of your mind.
Further, you almost *never* want to do any serious shell programming in csh or tcsh.
Why? Because csh and tcsh have an incomplete programming spec. Most notably are the way that it handles pipe data and the standard-output/error file descriptors. Specifically, you cannot throw away stdout while keeping stderr. This short-coming may not be noticeable to all programmers that choose csh/tcsh, but if you want to do any serious programming, you'll eventually hit those limitations and be forced to move to a real shell (real in the sense that it has a complete programming specification).
I personally never recommend csh/tcsh as a scripting language… but I can see the benefit that certain constructs (like "repeat N cmd") have, purely for their simplicity (and readability for *very* short scripts). To challenge myself on this topic, I routinely try and cross-port very complex shell scripts to csh, and while I can do math with "let" and I can other things that a *normal* scripting language should allow… I invariably end up running away screaming in frustration.
csh != tcsh != sh != bash != ash != dash != zsh
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