Bash - redirect to file (bug)?

Question

I am baffled by this example:

[ziga@brane ~]$ touch test.txt
[ziga@brane ~]$ echo "TEST" > test.txt
[ziga@brane ~]$ cat test.txt
TEST
[ziga@brane ~]$ cat test.txt | grep "TEST"
TEST
[ziga@brane ~]$ cat test.txt | grep "TEST" > test.txt
[ziga@brane ~]$ cat test.txt
[ziga@brane ~]$

Why does cat test.txt | grep "TEST" > test.txt delete contents of a file test.txt? I thought that > rewrites and >> appends! What am I missing?

Also covered in Warning regarding ‘>’. – G-Man Says 'Reinstate Monica' Dec 06 '20 at 20:59 — G-Man Says 'Reinstate Monica', Dec 06 '20 at 20:59

score 1 · Answer 1 · answered Dec 06 '20 at 20:59

1

because redirection happens first >test.txt so it truncate the file at very first, next cat test.txt read nothing and nothing pass to | grep "TEST" and so then nothing matches and finally nothing writes to opened file test.txt

answered Dec 06 '20 at 20:59

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Bash - redirect to file (bug)?

1 Answers1