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Full Discussion: Eval
Top Forums Shell Programming and Scripting Eval Post 302801739 by Corona688 on Thursday 2nd of May 2013 01:00:00 PM
Old 05-02-2013
If you don't understand why it works, then you've probably set yourself up for a surprise down the line when some kind of input you weren't expecting breaks it. eval is finicky that way. After all you're not feeding data into the shell, you're feeding it entire shell statements, and an unexpected backtick, pipe, semicolon, dollar sign, single quote, double quote, backslash will be interpreted by the shell as part of a statement and cause syntax errors or -- worse -- execute something unintended.

For this reason eval is to be avoided. It's difficult to safely control.

The answer to your question is simple enough. What, precisely, does it print when you run echo instead of eval?

Code:
echo echo "\" `cat myfile.file`\""

If it prints anything but a completely 100% syntactically-correct shell statement, then eval won't like it. If it's a valid shell statement, eval will run it. Nothing mysterious about it.

It's going to print something like
Code:
echo "line1
line2
line3
line4
line5
line6"

...which, as it happens, is a valid shell statement for printing those lines. Feed that into eval and it faithfully runs echo to print:
Code:
line1
line2
line3
line4
line5
line6

Correct me if I'm wrong, but that's not what you actually wanted, and why we've been shouting at you to ditch the useless echo.

If your file happens to contain a " character anywhere, this will be a huge problem for you, because eval will take it as an end-of-string which will cause a syntax error.

Last edited by Corona688; 05-02-2013 at 02:09 PM..
 

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ATF-SH(1)						    BSD General Commands Manual 						 ATF-SH(1)

NAME
atf-sh [-s shell] -- interpreter for shell-based test programs SYNOPSIS
atf-sh script DESCRIPTION
atf-sh is an interpreter that runs the test program given in script after loading the atf-sh(3) library. atf-sh is not a real interpreter though: it is just a wrapper around the system-wide shell defined by ATF_SHELL. atf-sh executes the inter- preter, loads the atf-sh(3) library and then runs the script. You must consider atf-sh to be a POSIX shell by default and thus should not use any non-standard extensions. The following options are available: -s shell Specifies the shell to use instead of the value provided by ATF_SHELL. ENVIRONMENT
ATF_LIBEXECDIR Overrides the builtin directory where atf-sh is located. Should not be overridden other than for testing purposes. ATF_PKGDATADIR Overrides the builtin directory where libatf-sh.subr is located. Should not be overridden other than for testing purposes. ATF_SHELL Path to the system shell to be used in the generated scripts. Scripts must not rely on this variable being set to select a specific interpreter. EXAMPLES
Scripts using atf-sh(3) should start with: #! /usr/bin/env atf-sh Alternatively, if you want to explicitly choose a shell interpreter, you cannot rely on env(1) to find atf-sh. Instead, you have to hardcode the path to atf-sh in the script and then use the -s option afterwards as a single parameter: #! /path/to/bin/atf-sh -s/bin/bash ENVIRONMENT
ATF_SHELL Path to the system shell to be used in the generated scripts. SEE ALSO
atf-sh(3) BSD
September 27, 2014 BSD
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