sort file.txt > file.txt is empty

Question

I have a doubt about a simple command and it is because in bash if I do

sort file.txt > file.txt

file .txt remains empty (it shouldn't keep file.txt with what it had before but sorted) but if I do

sort file.txt >> file.txt

then the ordered elements are added to the previous elements.

duplicates: [why does this blank the file?](https://stackoverflow.com/q/16634207/995714), [Why does redirecting the output of a file to itself produce a blank file?](https://superuser.com/q/597244/241386), [How can I use a file in a command and redirect output to the same file without truncating it?](https://stackoverflow.com/q/6696842/995714), [Why redirecting output sometimes produces an empty file?](https://unix.stackexchange.com/q/586443/44425), [Why is the file being emptied during input/output redirection to the same file?](https://askubuntu.com/q/1289971/253474) — phuclv, Apr 23 '22 at 16:10

Paul Hodges · Answer 1 · 2022-04-24T13:22:36.200

1

As soon as you targeted your file with > the shell truncated all the data before sort could read it.

sort file.txt > tmp.txt && mv tmp.txt file.txt

edited Apr 24 '22 at 13:22

answered Apr 23 '22 at 16:06

Paul Hodges

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it's the **shell** who does the redirection and truncation. The OS has nothing to do with it – phuclv Apr 23 '22 at 16:07
Valid - Edited. – Paul Hodges Apr 24 '22 at 13:22

score 1 · Accepted Answer · answered Apr 23 '22 at 16:07

This is the expected behavior. The phrase > file.txt opens file.txt for overwriting before the command sort is even run. So when sort runs it sees that file.txt is empty.

On the other hand, the phrase >> file.txt opens file.txt for appending before the command sort is even run. So when sort runs it reads the full file and then appends the results to the file.

sort file.txt > file.txt is empty

2 Answers2