Echo to file descriptor overwrites the file?
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
New contributor
add a comment |
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
New contributor
add a comment |
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
New contributor
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
New contributor
New contributor
edited 52 mins ago
Community♦
1
1
New contributor
asked 3 hours ago
ZoonoseZoonose
83
83
New contributor
New contributor
add a comment |
add a comment |
1 Answer
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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
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
add a comment |
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1 Answer
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oldest
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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
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
add a comment |
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
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
add a comment |
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
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
answered 1 hour 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
add a comment |
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
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
add a comment |
Zoonose is a new contributor. Be nice, and check out our Code of Conduct.
Zoonose is a new contributor. Be nice, and check out our Code of Conduct.
Zoonose is a new contributor. Be nice, and check out our Code of Conduct.
Zoonose is a new contributor. Be nice, and check out our Code of Conduct.
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