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Top Forums Shell Programming and Scripting Shell quoting rules for other languages programmers Post 302715553 by wyderkat on Monday 15th of October 2012 05:44:29 AM
Old 10-15-2012
Shell quoting rules for other languages programmers

I've seen so many times that programmers were confused about shell quoting and white spaces interpretation, that I decided to investigate that problem deeper. And my conclusion is that quoting in shells is very different from other programming languages. Programmers who have bigger experience in C/Python/Java/PHP than in Shell should remember about two unintuitive rules:
  • everything in shell is a string, and one need to quote everything properly. Also when reading variables: "$X", not only storing X="hi there"
  • shell will parse lines of code just one time, leaving unresolved nested variables and commands. In this case programmer need to explicitly use "eval" function.

I've written short article about this if anyone will be more curious you can find it under "white shell cofoh"
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ShellQuote(3)						User Contributed Perl Documentation					     ShellQuote(3)

NAME
String::ShellQuote - quote strings for passing through the shell SYNOPSIS
$string = shell_quote @list; $string = shell_quote_best_effort @list; $string = shell_comment_quote $string; DESCRIPTION
This module contains some functions which are useful for quoting strings which are going to pass through the shell or a shell-like object. shell_quote [string]... shell_quote quotes strings so they can be passed through the shell. Each string is quoted so that the shell will pass it along as a single argument and without further interpretation. If no strings are given an empty string is returned. If any string can't be safely quoted shell_quote will croak. shell_quote_best_effort [string]... This is like shell_quote, excpet if the string can't be safely quoted it does the best it can and returns the result, instead of dying. shell_comment_quote [string] shell_comment_quote quotes the string so that it can safely be included in a shell-style comment (the current algorithm is that a sharp character is placed after any newlines in the string). This routine might be changed to accept multiple string arguments in the future. I haven't done this yet because I'm not sure if the strings should be joined with blanks ($") or nothing ($,). Cast your vote today! Be sure to justify your answer. EXAMPLES
$cmd = 'fuser 2>/dev/null ' . shell_quote @files; @pids = split ' ', `$cmd`; print CFG "# Configured by: ", shell_comment_quote($ENV{LOGNAME}), " "; BUGS
Only Bourne shell quoting is supported. I'd like to add other shells (particularly cmd.exe), but I'm not familiar with them. It would be a big help if somebody supplied the details. AUTHOR
Roderick Schertler <roderick@argon.org> SEE ALSO
perl(1). perl v5.16.3 2010-06-11 ShellQuote(3)
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