Shell Programming and Scripting

Originally Posted by bakunin

[..]
Now back to your original line:

cmd='ls -l *.TXT'
$cmd

With the first line (the single quotes) we told the shell to switch off field splitting for the string, so it is ONE string: ls -l *.TXT. This "holds" for exactly one evaluation of the string, therefore a command named ls -l *.TXT (literally!) is searched for - but perhaps not found.

You can try for yourself, execute this in your home dir:

Code:

cat - > ./"ls -l foo" <<EOF
#! /bin/sh

echo "here is the awkward command"
exit 0
EOF
chmod 754 "./ls -l foo"

you now have a file awkwardly named ls -l foo in your current directory. Now execute this:

Code:

cmd='ls -l foo'
./$cmd

And you will see that in fact this script is called. This in turn means that the whole string, including the whitespace, was intepreted as one field. Remove this script with the command rm "./ls -l foo".

This is all because the quoting protected the string from being subjugated to the field splitting process for one time. Notice, that if the string is interpreted a second time, this protection wears off:

Code:

cmd='ls -l foo'
eval $cmd

eval is a keyword which restarts the whole parsing process (and hence the field splitting) of the shell when reading this line. The line is interpreted twice and because of this the field splitting takes place (but only on the second pass, as some more intricate examples will show you - you may want to experiment and find out how to prevent/enforce this).

I hope this helps.

bakunin

Each variable assignment shall be expanded for tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal prior to assigning the value.

The quote characters ( <backslash>, single-quote, and double-quote) that were present in the original word shall be removed unless they have themselves been quoted.