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Shell Scripting - how to get command successful result

Posted on 2008-10-16
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Last Modified: 2013-12-06
Hi,

I am trying to execute a touch command (am open to any other commands as well). In the shell script I want to perform something like this:

IF [ touch file1.txt ]
 THEN
      // File didn't exist but it has been created now
ELSE
      // File already exists
END

Now I know i can do, IF file_exists but I want to touch a file and ensure that very instant when I touched the file was created and it didn't exist before. Hence if I can't do IF file_exists and then touch.

I want to execute a command (any command, please suggest) that will create the file and along with that return a result to a IF statement informing it about the file creation, else if the file existed then it must not be recreated.

I know this is a little convoluted explanation but it would really help if you can answer this, please ask any clarification questions as well.
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Question by:infa007
6 Comments
 
LVL 40

Expert Comment

by:omarfarid
ID: 22737230
try

if [ -f file1.txt ]
then
      echo file exists
else
      touch file1.txt
      echo file created
fi

0
 

Author Comment

by:infa007
ID: 22737353
Thanks, thats exactly what I don't want to do ... consider a php programming solution ...

function abc($x) {
 
if($x == 1)
 return 0;
else
 return 1;
}

// Here is what I want to do from Unix

if(abc(2)) {
// received 1
}
else {
// received 0
}

Instead of calling abc function I want to, call a command, e.g. touch, and require it to let me know if the touch of the file was successful (while not overwriting the file)
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Expert Comment

by:omarfarid
ID: 22738530
touch does not over write any file. it can be used to change the creation / modification time.
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LVL 68

Expert Comment

by:woolmilkporc
ID: 22738561
But touch doesn't indicate (by returncode) whether the file already existed or not. I think, such a functionality is what infa007 is searching for.
I, personally, don't know any utility with that functionality.
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Expert Comment

by:omarfarid
ID: 22739005
that's why there is an if then
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Accepted Solution

by:
sjm_ee earned 500 total points
ID: 22741921
The questioner states that "ensure that very instant when I touched the file was created and it didn't exist before". So what the questioner seems to be trying to do is to use the file as a semaphore, presumeably to create a critical section within the code so that it is only executed once at a time.

If this is correct, then you should understand that file creation and deletion are atomic operations under AIX, UNIX and many OSes, particularly those with journalled file systems. However the atomic file system equivalent of "test and set" is not available from any command that I am aware of. You can get this behaviour in C with "open(O_CREAT|O_EXCL)".

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