Home / ubuntu / Questions / 1167530
Accepted
Hooman Bahreini
Asked: 2019-08-22 21:10:46 +0800 CST

understanding bash redirection using > char

  • 772

I am learning bash and am not able to understand what is going wrong with the output redirection in the following example:

I have a file called myfile.txt with the following content.

Practice makes Perfect

I am going to use tr command to replace P with p:

cat myfile.txt | tr P p

This does what I want, now I am going to put the result back into the original file:

cat myfile.txt | tr P p > myfile.txt

But after executing the above command myfile.txt is empty... why is this happening?

Update:

If I send the output to a different file, then it works as expected:

cat myfile.txt | tr P p > anotherfile.txt
command-line bash redirect

3 Answers

  1. Best Answer

    Bash (and other POSIX-compliant shells) follow two rules that explain this behavior:

    • Redirection happens before commands run.

    • If a file exists, it is truncated (emptied) when redirection goes to the file as in > myfile.txt.

    So because there is > redirection the contents of the myinput.txt get discarded and cat prints nothing, cat exits and closes the pipe and tr closes the file without writing anything to it. Order in which commands start is not particularly important ( and it is not guaranteed, see Gille's answer here). But what is important is that redirection occurring before commands starting; that is each command's standard streams are prepared, and that's what causes empty file in the end.

    But it is possible to edit the same file tools like sponge, just not in this particular case.

    See also:

    • 2

  2. Use: sort -o instead of redirection. I n such a way you will be able to write to the same file.

    cat myfile.txt | tr P p | sort -o myfile.txt
    • 0

  3. In my opinion cat is not used in the correct intention. Normally cat is used to combine files, but often also misused to just use display simple files.

    In your case I would use the tr command in the following way:

    tr P p < myInput.txt > myOutput.txt
    • -1