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Accepted
favarjooy86
Asked: 2015-04-14 12:01:26 +0800 CST

sorting a file using the sort command and overwrite it

  • 772

i was trying to sort a text file and replace the sorted file with the original one, so i set the file to be also the output of the sort command too, but when i do that it clears the file contents and the file becomes empty, why? i can use another file to solve this problem but it doesn't feel nice :D, ( and i'm using Lubuntu. )

faav@faav-XPS-L501X:~$ cat test
A
C
B
faav@faav-XPS-L501X:~$ ls -l test
-rw-rw-r-- 1 faav faav 6 Apr 14 00:28 test
faav@faav-XPS-L501X:~$ sort test > test
faav@faav-XPS-L501X:~$ cat test
faav@faav-XPS-L501X:~$ ls -l test
-rw-rw-r-- 1 faav faav 0 Apr 14 00:28 test
sort

3 Answers

  1. Best Answer

    Use the -o option of sort instead of using redirection operator, this will edit the file in place.

    From man sort:

    -o, --output=FILE
              write result to FILE instead of standard output

    Here is a test:

    $ cat test.txt 
A
C
B

$ sort test.txt 
A
B
C

$ sort test.txt -o test.txt

$ cat test.txt 
A
B
C
    • 13

  2. This happens because the input / output file is truncated before its content is actually read, so sort ends up reading an already empty file.

    In general (i.e. while redirecting the output of a command to its input file, since this issue doesn't only happen with sort), you can execute the required command within a subshell and store its output to a variable, then flush the content of the variable to the file (e.g. with echo), this way delaying the truncation of the input / output file to a point where the input file has been read already:

    tmp=$(sort test) && echo "$tmp" >  test

    Also note that this is not very efficient for large files, and that the && operator has been chosen in place of ; to avoid the truncation of the input / output file in case of a failure of the first command.

    • 1

  3. You may write like sort -b -o filename filename, where filename specifies the same file twice as you want the output to be saved or overwrite to the original file. This command works like, it shall remove any blank spaces and the sort the contents of file and overwrite to original file.

    Then you can print the contents of file by mentioning cat filename, to print the contents of file as output on standard output device.

    Thanks.

    • -1