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Question related to bash. Your help needed.

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Top Forums Shell Programming and Scripting Question related to bash. Your help needed.
# 8  
Old 09-15-2016
Quote:
Originally Posted by Corona688
What you are asking for is a very persistently repeated beginner shell programming mistake called "Dangerous Backticks".
Quote:
(From the URL above)

A special idiom to pay attention to, because it's basically always wrong, is this:
Code:
    	for f in `cat file`; do
    	    ...
    	done

Apart from the classical Useless Use of Cat here, the backticks are outright dangerous, unless you know the result of the backticks is going to be less than or equal to how long a command line your shell can accept. (Actually, this is a kernel limitation. The constant ARG_MAX in your limits.h should tell you how much your own system can take. POSIX requires ARG_MAX to be at least 4,096 bytes.)
It wastes time, memory, external calls, and subshells to slowly and unreliably almost do what you wanted. The read shell builtin actually reads lines, running externals in backticks do not -- they split on any spaces at all.
I fully agree that using a command substitution here wastes time, memory, external calls, and subshells; and splits on words instead of lines; but, ARG_MAX doesn't come into play here. ARG_MAX applies to the size of the argument lsat and environment list of a command that is executed, it does not apply to the amount of the data written by that command. Since the shell doesn't exec a for loop (it runs it in the current shell execution environment), ARG_MAX doesn't apply when gathering arguments for a for loop.
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