Bashscripts.org

All,

I'm relatively new to this and probably do a lot of things in very backwards and incorrect ways, but any help that could be offered would be greatly appreciated.

I have two scripts that I use to update Linux machines remotely, the first is a scheduled to a cron job that runs every hour, and it wget's the second file which actually contains the patch, runs it, and then deletes it. It also directs the output of the patch to a file.

The tricky part comes in the patch itself, as it uploads the output of itself to a website(I know...). The upload command is at the end of the patch, but the upload is still failing due to the output not being written yet.

So, is it possible to have the output write to the file as it occurs rather then letting the script finish? I can provide any code needed if this is to vague or just doesn't make sense.

Thanks in advance,

-Tim

Just put everything except the 'upload' part of your script between curly brackets and redirect the output to a logfile. Then as a last line do your 'upload'. (and/or delete the logfile)

grrr, there is no good reason for using cat in bash scripts!

It was meant as an example, so he could replace the 'cat' command with whatever he's using to post the logfile with.

You have to be taking the piss, if all you want to do is output the file to screen then cat is fine. How do the extra 7 characters in your example improve function, clarity or beauty?

man bash says :

That's what the man page says but it does not say that 'echo "$(<output.log)"' is faster than 'cat output.log' when all you want to do is output the data, and the cat command is much clearer.

It is true that you can use whatever method you think is best but I personally would use 'cat file' unless I was doing further processing on the data. I think it's easier to understand, not only for beginners but also for others reading your script

Of course in the actual example given earlier I would have used tee instead

I would have let him pipe the output directly to his 'upload' command as well, if I was sure that there wouldn't be any other commands in between.

calling an external command is always slower.

You should be careful what you preach

indeed.

but, the second is faster because it doesn't open the file each time.

But that's cheating! The second scripts only reads the contents of the file once and prints it 10000 times. Not read and print the file 10000 times, which is what 'cat' does. And we were talking about that

he he he, well.... While I would agree that cat is used way more than necessary.... I'd disagree that there is no good reason to use cat in a bash script...
Here are some.

An extremely simple method to write to a new file.
cat > somefile
<enter information here>
<when done CTRL-D>

Create an actual line numbered copy of your file
cat -n somefile num_somefile

View special characters that might be in a file
cat -v somefile
<this shows CTRL chars etc>

The normal use
cat file1 file2 file3 > newfile

print file1 and file2 to standard output
cat file1 file2


And for clarity.... I ALWAYS use
cat filename | sed/awk/grep .....especially if I have multiple pipes... it's just easier to read. I understand it isn't technically correct... but it is much clearer to read, not to mention less to type.

Purists tend to stay away from using cat, they also tend to stay away from sed/awk and use bash parameter substitution too. But neither make it easier to understand what the program is actually doing. As a systems administrator, I would prefer to see/read a script that is documented and easy to understand quickly...but that's just me

However, if I was writing something that was going to be released on sourceforge or something like that, I'd definitely tidy up my code