Shell Programming and Scripting

Hi,
I am new in unix. I found one while command as while [ ! -z "$SRC_FILE" ].
It would be really help if you could let me know what is the use of "-z" and what is difference between "-n" and "-z". Thanks for your help!!

Please use code tags as required by forum rules!

Reading man bash (or whatever shell you use) would help. Paragraph on CONDITIONAL EXPRESSIONS:

Thanks RudiC!!

Code:

export SRC_FILE = ls -at xxx_yyy_* |tail -n 1
   while [ ! -z "$SRC_FILE" ]
      do
            mv $SRC_FILE `date +%Y%m%d_%H%M%S`_xxx_yyy.txt
            sleep 10

           SRC_FILE=`ls -at xxx_yyy_* |tail -n 1`
      done

in this case what will happen if there are five files with name 'xxx_yyy_*'?
will it rename all or only the first one?
I think in our case, only first file gets renamed and others are untouched.
thanks for help.

Moderator's Comments:

Please use CODE tags (not ICODE tags) for multi-line input, output, and code segment samples.

This is an overcomplicated approach to moving n files. You repeatedly list ALL files and then tail only the last - which is changing in every iteration as the former one has been moved. Some comments:
- no need to export - var is only used locally
- no need to use the -at options to ls
- the file_to_process.txt is not used anywhere
- the date stamp is evaluated in every iteration and will be changing between loops. Oh, I see, it's used to differentiate between files. Is that wise?
Anyhow, how about

Code:

ls xxx_yyy_* |
while read FN
  do DATESTAMP=$(date...)
     mv "$FN" $ARCHIVE_PATH/$DATESTAMP_xxx_yyy.txt
  done

Here is a slightly different take on what RudiC has already said about the above script...

There are several problems with this script (including, but not necessarily limited to):Perhaps, something like this would better suite your needs:

Code:

for SRC_FILE in xxx_yyy_*
do	if [ -f "$SRC_FILE" ]
	then	mv "$SRC_FILE" "$(date '+%Y%m%d_%H%M%S')_xxx_yyy.txt"
		sleep 1
	fi
done

The difference between this script and RudiC's suggestion is that the above script will run a little faster (since it doesn't execute the ls command) but it won't generate a diagnostic message if there are no files with names starting with xxx_yyy_ when you start the script (although one could be added as an else clause in the if statement. If there are files matching your pattern that are not regular files, this script will ignore them while RudiC's script will move them. And, since there is no delay between moves in RudiC's script, files could be overwritten if two or more files are moved within the same wall clock second. (The sleep 1 in the loop in my code avoids that problem. I don't see any need for the 10 second delay you had in your loop.)

And, finally to answer your question: "what will happen if there are five files with name 'xxx_yyy_*'?" I would expect a syntax error from the export command that will leave SRC_FILE undefined. So the expansion of $SRC_FILE in the while loop will be a zero length string and the commands in the while loop will never be executed unless your script inherits a value for that variable in its environment. If a value is inherited in the environment, zero or more files not matching your pattern could be moved or could cause a diagnostic message to be printed if mv can't move a file or several files (since you didn't quote the expansion in the mv command) named by that variable, and then the script would go on to do what you want because you reset that variable at the end of the loop each time you go through the loop.