Shell Programming and Scripting

sorry for the late reply, just been busy work and all

i have added to my script if the tar file already exists in the "archived_projects" folder do not over write and also the size and file count

Code:

#!/bin/bash
cd /to_be_archived/
for DIR in * ;

    do
    MailAddress="robertw@molinare.co.uk"

tar -cf "$DIR".tar "$DIR"

    if [ $? -ne 0 ]
    then
    mail -s "tar failed $DIR" $MailAddress <<< "creating of tar $DIR failed due to error, removing $DIR.tar"
    rm -f "$DIR".tar
    continue
    fi

    if [ -f /archived_projects/"$DIR".tar ]; then
      mail -s "duplicate exists $DIR" $MailAddress <<< "$DIR.tar already exists"
    rm -f "$DIR".tar
    continue
    fi

rsync -a "$DIR".tar /archived_projects/

    if [ $? -ne 0 ]
    then
    mail -s "rsync failed $DIR" $MailAddress <<< "rsync of $DIR failed due to error, removing $DIR.tar"
    rm -f "$DIR".tar
    continue
    fi

rm -f "$DIR".tar

    if [ $? -ne 0 ]
    then
    mail -s "remove tar failed $DIR" $MailAddress <<< "removing of tar $DIR failed due to error"
    continue
    fi

du -h "$DIR" >> /archive_details
ls -1 /to_be_archived/"$DIR" | wc -l >> /archive_details

rm -rf "$DIR"

    if [ $? -ne 0 ]
    then
    mail -s "remove folder failed $DIR" $MailAddress <<< "removing of $DIR failed due to error"
    continue
        else
        mail -s "success $DIR" $MailAddress <<< "successfully completed archiving $DIR"
    fi

    done

---------- Post updated at 08:27 AM ---------- Previous update was at 06:10 AM ----------

i think thats a good idea condensing the remove commands to one, i will do that in my next revision, but what bakunin said about you dont need to add the return code to the if statement, i dont seem to get it as for every command you type if it returns 0 (success) or 1-255 (failed) it will either do then or the else statement

but you havnt put in any return code so how does it know whether the command before has succeeded or failed?

OK, short introduction to the intractes of "if":

"if" is followed by a (any) command and it executes first this command, then the "then...."-part if the command returns RC 0 or the "else..."-part, if not. This is always the case. For instance:

Code:

if some_command ; then
     echo "some_command returned 0"
else
     echo "some_command returned something else"
fi

Here, the command "some_command" is executed. If it returns 0 the then-part is executed, if it returns something else the "else"-part is executed.

You may ask yourself where this:

Code:

if [ $? -eq 0 ] ; then

now comes from because there seems no command to be involved: wrong. In fact there is a command involved and it is called "test". "test" does all kinds of tests: integer comparisons, string comparisons and tests for the existence (or certain attributes) of files and it returns a RC of 0 for the comparisons being (logically) true and an RC <> 0 for comparisons being logically false.

Historically, in the first UNIX versions the construct looked like this (and you may see this still in some very old scripts):

Code:

if test $? -eq 0 ; then

or maybe:

Code:

if /usr/bin/test $? -eq 0 ; then

You see, the expression to be evaluated consists just of arguments to "test". "test", is still there, i suggest to read the man page of it.

But because this didn't look like the programming languages the inventors of UNIX knew (they were all programmers, after all), they created a device which made it look more like what they were accustomed to: they created a symbolic link from /usr/bin/test to /usr/bin/[ - in fact "[" is a possible filename - and the line now looked like this:

Code:

if [ $? -eq 0 ; then

In some variants of UNIX (HP-Ux, if i remember right) this link still exists.

That was actually better but it looked like a bracket was opened and never closed. So the final change to /usr/bin/test was that it expected a "]" as the last argument if it was called as "[" - and this created the look we know today:

Code:

if [ $? -eq 0 ] ; then

Notice, that for this reason the following won't work:

Code:

if [ $? -eq 0] ; then

because the last parameter now would be "0]", and the expected final "]" would be missing. Try it, you will see that it leads to a syntax error - for the reasons stated above. The following weird-looking construct, though, works perfectly:

Code:

if [ "$?" "-eq" "0" "]" ; then

The last step in the development was that programmers of UNIX shells (and i think David Korn with his Korn shell was the first) noticed that "test" and its equivalent was called so often that they made it a shell built-in. This will prevent the necessity of loading and starting an external executable (time-consuming activities) and is a speed optimisation. Still, the buil-in "[" works the same as the external /usr/bin/test (again, with the exception of expecting "]" as the last argument) as the man page for the shell will tell you.

I hope this helps.

bakunin

The POSIX standards require that both test and [ (and all other standard utilities except special built-ins) exist as stand-alone utilities (not just as shell built-ins). This allows them to be used as command arguments to the env, find -exec, nice, nohup, time, and xargs utilities.

The [ expression ] form of the test utility was actually added to UNIX systems before symlinks. On many UNIX and UNIX-like systems, the test and [ utilities are hard links to the same executable file instead of one of them being a symlink or both of them being separate executables.

so am i right in thinking -

Code:

tar -cf "$DIR".tar "$DIR"
if [ $? -ne 0 ]
then
mail -s "tar failed $DIR" $MailAddress <<< "creating of tar $DIR failed due to error, removing $DIR.tar"
rm -f "$DIR".tar
continue
fi

is the same as this -

Code:

if tar -cf "$DIR".tar "$DIR" ; else
mail -s "tar failed $DIR" $MailAddress <<< "creating of tar $DIR failed due to error, removing $DIR.tar"
rm -f "$DIR".tar
continue
fi

as you said then is equal to 0 and else is not equal to 0?

many thanks

rob

No! You can't have an if statement without a then clause. But, you can do it as I suggested back in post #16 in this thread:

Code:

	if ! tar cf "${DIR}".tar "${DIR}"
	then	mail -s "tar failed $DIR" $MailAddress <<< "creating of tar $DIR failed due to error, removing $DIR.tar"
		rm -f "$DIR".tar
		continue
	fi

Note the exclamation point in red in the if statement. That reverses the exit status of the given command (tar in this case) so that if the command fails (exits with a non-zero exit status) it will execute the then clause and if the command succeeds (i.e., exits with zero exit status) it will execute the else clause if there is one.

Ohh so an ! Means not equal to 0

No. The ! keyword in the shell doesn't MEAN anything. It DOES something. As I said before, it "reverses the exit status of the given command (tar in this case) so that if the command fails (exits with a non-zero exit status) it will execute the then clause and if the command succeeds (i.e., exits with zero exit status) it will execute the else clause if there is one.

Look at each of the following if statements and try to determine what the output will be. Then run the command and see if it did what you expected:

Code:

if true; then echo then; else echo else; fi

if ! true; then echo then; else echo else; fi

if ! ! true; then echo then; else echo else; fi

if false; then echo then; else echo else; fi  
 
if ! false; then echo then; else echo else; fi

if ! ! false; then echo then; else echo else; fi