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Old 10-07-2014, 07:45 AM   #1
sumncguy
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Registered: Jun 2013
Posts: 71

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Interesting Behavior


Im taking a Linux Essentials class at a local community college. Ive been using the command line for some time in one nix environment or another. Im ok .. but I want to fill in the blanks and LPI cert. Anyway ... this was on a lab. The professor didn't go into detail .. and ironically no other student questioned what was going on...

Quote:
[popeye bin]$ ls -l > file.txt

[popeye bin]$ cat file.txt
total 8
-rw-rw-r-- 1 popeye popeye 16 Oct 7 07:38 file1.txt
-rw-rw-r-- 1 popeye popeye 17 Oct 7 07:38 file2.txt
-rw-rw-r-- 1 popeye popeye 0 Oct 7 07:38 file.txt
Notice the file size of file.txt

Quote:
[popeye bin]$ ls -l file.txt
-rw-rw-r-- 1 popeye popeye 178 Oct 7 07:38 file.txt

[popeye bin]$
I had to think about this for a few minutes.

It looks like the file is created and ls is executed, before the file is written to...

Is there some order of operation going on ?
 
Old 10-07-2014, 08:01 AM   #2
linosaurusroot
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Registered: Oct 2012
Distribution: OpenSuSE,RHEL,Fedora,OpenBSD
Posts: 982
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Your shell is doing this: you are right that creation of the output happens before ls produces the listing.

Code:
receive command from user
fork into two (almost identical but nevertheless distinguishable) processes
  child process:
    redirect stdout to file.txt (creating new empty file)
    execute "ls -l" with stdout as defined above
    and end ... nothing follows successful execution (of ls)

  parent process:
    wait for the child process to end (and observe exit status)
    prompt user for next input
 
  


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