Shell Scripting Resources && Conditional

Question

Hiya folks.
I'm looking for a great shell scripting site. Or a book. The Apple Mac Dev Centre is ... well she ain't workin for me.
For instance, I'm looking for some leadership on a conditional, which those docs won't show me:
troubleshooting=false
*if [$troubleshooting==1]; then*
* echo $troubleshooting*
* echo $nameStart*
* echo $myDate*
* echo $newFName*
* echo $mSource*
* echo $mTarget*
fi
How do I trip this conditional?
Cheers

Answer 1

How do you set a boolean?
Everything in a shell variable is a string. Some of those strings may look like numbers, but as far as the variable is concerned they are strings. There are no boolean variables. It is all in how you interpret the strings in your variable.
I'll have to check up on the syntax for bash.
You could read, reread, read and read "man bash". But I know it is not easy to actually figure out how to use the stuff the man page tells you, or even what they really mean when they use various terms. A book with examples helps, but there is magic hidden in the man page that sometimes takes ages and heavy use of shell scripting to figure out, and I've found that no one book will clearly explain every trick that can be done with shell scripts.
I prefer to have spaces in there, but not around parens of any kind. I wish languages would be the same that way. Heh.
And I like to have spaces around equal signs in assignments, but Bourne based shell scripts have their own rules. That is just the way it is.
The syntax for the 'if' command is
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
where "A list is a sequence of one or more pipelines separated by one of the operators ...". "A pipeline is a sequence of one or more commands separated by the character |".
In the original Bourne shell (which ran in 64kilobytes of memory for code, data, and stack space) all expressions were handed by an external program 'test'. So your 'if' statement would have originally been
if test $troubleshooting = 1; then
fi
Someone figured out that Unix filenames do not care what characters make up their name, so a hardlink was created between /bin/test and /bin/[
ls -li /bin/test /bin/[
57880389 -r-xr-xr-x 2 root wheel 46720 Jun 17 2009 /bin/[
57880389 -r-xr-xr-x 2 root wheel 46720 Jun 17 2009 /bin/test
You will notice that 57880389 (the file's inode) is identical for both 'test' and '['.
So when you wrote
if [$troubleshooting==1]; then
the shell first processed the line and performed variable substitution, so the 'if' line converts to
if [false==1]; then
now the shell processes the 'list' of pipelines/commands following the 'if' statement. So a command is the first white space separate token, which is "[false==1]", so the shell looked in all the PATH directories looking for a file with the name "[false==1]" and did not find anything.
However, if you white space separate the command in the 'list' from its arguments, then the shell can figure out what the first command is, invoke that command, and pass its arguments to the command.
Now shells have more than 64kilobytes of memory to work with, and so the [[ ... ]] syntax was invented as a way to indicate that the expression was to use the shell built-in evaluation code. And I think bash eventually decided that it would do the /bin/[ program code evaluation as a built-in as well.
But since you can also say
if myprogram --opt --opt arg arg arg; then
fi
the shell must still be able to tell the difference between [ or [[ as expression evaluation operators, and a legal file with the name /home/me/bin/[myprogram, or $HOME/bin/[[myprogram, so it cannot assume that [ or [[ always start an expression evaluation.