Echo to file descriptor overwrites the file?












1















I am having trouble understanding what is happening when I try to write to a file descriptor? It appears to be overwriting the original contents? Is this expected behaviour?



I have replicated this in the example below:



$ echo "The quick brown fox ..." > example.txt  

$ echo "The quick brown fox ..." >> example.txt

$ cat example.txt

The quick brown fox ...  

The quick brown fox ...

$ exec 88<>example.txt

$ cat example.txt

The quick brown fox ...  

The quick brown fox ...

$ echo "jumped" >&88  

$ cat example.txt

jumped  

ck brown fox ...  

The quick brown fox ...

$ echo "jumped" >&88  

$ cat example.txt

jumped  

jumped  

n fox ...  

The quick brown fox ...





bash files exec







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    1















    I am having trouble understanding what is happening when I try to write to a file descriptor? It appears to be overwriting the original contents? Is this expected behaviour?



    I have replicated this in the example below:



    $ echo "The quick brown fox ..." > example.txt  
    
$ echo "The quick brown fox ..." >> example.txt
    
$ cat example.txt
    
The quick brown fox ...  
    
The quick brown fox ...
    
$ exec 88<>example.txt
    
$ cat example.txt
    
The quick brown fox ...  
    
The quick brown fox ...
    
$ echo "jumped" >&88  
    
$ cat example.txt
    
jumped  
    
ck brown fox ...  
    
The quick brown fox ...
    
$ echo "jumped" >&88  
    
$ cat example.txt
    
jumped  
    
jumped  
    
n fox ...  
    
The quick brown fox ...





    bash files exec







    share|improve this question









    New contributor




    Zoonose is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      1












      1








      1


      3






      I am having trouble understanding what is happening when I try to write to a file descriptor? It appears to be overwriting the original contents? Is this expected behaviour?



      I have replicated this in the example below:



      $ echo "The quick brown fox ..." > example.txt  
      
$ echo "The quick brown fox ..." >> example.txt
      
$ cat example.txt
      
The quick brown fox ...  
      
The quick brown fox ...
      
$ exec 88<>example.txt
      
$ cat example.txt
      
The quick brown fox ...  
      
The quick brown fox ...
      
$ echo "jumped" >&88  
      
$ cat example.txt
      
jumped  
      
ck brown fox ...  
      
The quick brown fox ...
      
$ echo "jumped" >&88  
      
$ cat example.txt
      
jumped  
      
jumped  
      
n fox ...  
      
The quick brown fox ...





      bash files exec







      share|improve this question









      New contributor




      Zoonose is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.












      I am having trouble understanding what is happening when I try to write to a file descriptor? It appears to be overwriting the original contents? Is this expected behaviour?



      I have replicated this in the example below:



      $ echo "The quick brown fox ..." > example.txt  
      
$ echo "The quick brown fox ..." >> example.txt
      
$ cat example.txt
      
The quick brown fox ...  
      
The quick brown fox ...
      
$ exec 88<>example.txt
      
$ cat example.txt
      
The quick brown fox ...  
      
The quick brown fox ...
      
$ echo "jumped" >&88  
      
$ cat example.txt
      
jumped  
      
ck brown fox ...  
      
The quick brown fox ...
      
$ echo "jumped" >&88  
      
$ cat example.txt
      
jumped  
      
jumped  
      
n fox ...  
      
The quick brown fox ...





      bash files exec




      bash files exec






      share|improve this question









      New contributor




      Zoonose is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question









      New contributor




      Zoonose is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|improve this question




      share|improve this question








      edited 1 hour ago









      Community♦

      1




      1






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      asked 3 hours ago









      ZoonoseZoonose

      83




      83




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      New contributor





      Zoonose is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






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      Check out our Code of Conduct.






















          1 Answer
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          3














          Because you hadn't done any reads on descriptor 88, the current seek position was "0", and so the write took place at that point.



          If, instead, you'd read the file before then, then appends happen:



          bash-4.2$ cat <&88
          
The quick brown fox ...
          
The quick brown fox ...
          

          
bash-4.2$ echo hello >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          

          
bash-4.2$ echo more >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          
more





          share|improve this answer
























          • Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

            – Zoonose
            1 hour ago











          • Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

            – Zoonose
            1 hour ago













          • @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

            – P Daddy
            6 mins ago











          Your Answer








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          1 Answer
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          1 Answer
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          active

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          active

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          3














          Because you hadn't done any reads on descriptor 88, the current seek position was "0", and so the write took place at that point.



          If, instead, you'd read the file before then, then appends happen:



          bash-4.2$ cat <&88
          
The quick brown fox ...
          
The quick brown fox ...
          

          
bash-4.2$ echo hello >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          

          
bash-4.2$ echo more >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          
more





          share|improve this answer
























          • Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

            – Zoonose
            1 hour ago











          • Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

            – Zoonose
            1 hour ago













          • @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

            – P Daddy
            6 mins ago
















          3














          Because you hadn't done any reads on descriptor 88, the current seek position was "0", and so the write took place at that point.



          If, instead, you'd read the file before then, then appends happen:



          bash-4.2$ cat <&88
          
The quick brown fox ...
          
The quick brown fox ...
          

          
bash-4.2$ echo hello >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          

          
bash-4.2$ echo more >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          
more





          share|improve this answer
























          • Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

            – Zoonose
            1 hour ago











          • Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

            – Zoonose
            1 hour ago













          • @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

            – P Daddy
            6 mins ago














          3












          3








          3







          Because you hadn't done any reads on descriptor 88, the current seek position was "0", and so the write took place at that point.



          If, instead, you'd read the file before then, then appends happen:



          bash-4.2$ cat <&88
          
The quick brown fox ...
          
The quick brown fox ...
          

          
bash-4.2$ echo hello >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          

          
bash-4.2$ echo more >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          
more





          share|improve this answer













          Because you hadn't done any reads on descriptor 88, the current seek position was "0", and so the write took place at that point.



          If, instead, you'd read the file before then, then appends happen:



          bash-4.2$ cat <&88
          
The quick brown fox ...
          
The quick brown fox ...
          

          
bash-4.2$ echo hello >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          

          
bash-4.2$ echo more >&88
          

          
bash-4.2$ cat example.txt 
          
The quick brown fox ...
          
The quick brown fox ...
          
hello
          
more






          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 2 hours ago









          Stephen HarrisStephen Harris

          26.1k24577




          26.1k24577













          • Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

            – Zoonose
            1 hour ago











          • Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

            – Zoonose
            1 hour ago













          • @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

            – P Daddy
            6 mins ago



















          • Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

            – Zoonose
            1 hour ago











          • Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

            – Zoonose
            1 hour ago













          • @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

            – P Daddy
            6 mins ago

















          Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

          – Zoonose
          1 hour ago





          Wow! So this means that the file descriptor (in this example 88) maintains a separate seek position to the standard cat? i.e. cat example.txt

          – Zoonose
          1 hour ago













          Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

          – Zoonose
          1 hour ago







          Thanks Stephen. Just tested this out and you are right. Interestingly enough, if I continue to echo out to the file again, and then echo to the descriptor (without reading on the descriptor) the problem persists. So if other processes are writing to the file, then I need to constantly read in on the descriptor, before writing out!

          – Zoonose
          1 hour ago















          @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

          – P Daddy
          6 mins ago





          @Zoonose: When you execute cat example.txt, cat opens the file, getting a descriptor with a seek position starting at the beginning of the file, reads it until the end, and then closes the descriptor. When you execute exec 88<>example.txt, bash opens the file, getting a descriptor with a seek position starting at the beginning of the file. The file itself doesn't maintain a seek position, instead a position is maintained for each open file descriptor.

          – P Daddy
          6 mins ago










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