CODEWITHSUNDEEP
linuxmacosunixcat
Billz
Billz

Reputation: 8105

command line "cat >> " behavior

What does this command do cat t.txt >> t.txt ? let say the t.txt only have one line of text "abc123". I assume the output of "abc123" is appended to t.txt. So, I should have 2 lines of "abc123". However, it just going in to a infinite loop. It doesn't stop until I hit Control-C. Is this the expect behavior of >>?

Upvotes: 0

Views: 554

Answers (3)

SheetJS
SheetJS

Reputation: 22925

cat program opens the file for reading, reads the file and writes to standard out.

>> is a shell append redirect.

What you are seeing is the following cycle:

  1. cat reads a line from t.txt
  2. cat prints the line to file
  3. the line is appended to t.txt
  4. cat tests if it is at the end of the file

That 4th step will always be false, because by the time the EOF check happens a new line has been written. cat waits because the write always happens first.

If you want to prevent that behavior, you can add a buffer in between:

$ cat t.txt | cat >> t.txt

In this way, the write occurs after cat t.txt checks for EOF

Upvotes: 4

UltraInstinct
UltraInstinct

Reputation: 44444

What you are trying to do by:

cat t.txt >> t.txt

is like telling your system to read t.txt line by line and append each line to t.txt. Or in better words, "append the file to itself". The file is being gradually filled up with repetitions of the original contents of the file -- the reason behind your infinite loop.

Generally speaking, try to stay away from reading and writing to the same file using redirections. Is it not possible to break this down to two steps -- 1. Read from file, output to a temporary file 2. append to the temporary file to the original file?

Upvotes: 2

Codie CodeMonkey
Codie CodeMonkey

Reputation: 7946

cat is a command in unix-like systems that concatenates multiple input files and sends their result to the standard output. If only one file is specified, it just outputs that one file. The >> part redirects the output to the given file name, in your case t.txt.

But what you have says, overwrite t.txt with the contents of itself. I don't think this behavior is defined, and so I'm not surprised that you have an infinite loop!

Upvotes: 0

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