Question

In cygwin, the following code works fine

$ cat junk
bat
bat
bat

$ cat junk | sort -k1,1 |tr 'b' 'z' > junk

$ cat junk
zat
zat
zat

But in the linux shell(GNU/Linux), it seems that overwriting doesn’t work

[41] othershell: cat junk
cat
cat
cat
[42] othershell: cat junk |sort -k1,1 |tr 'c' 'z'
zat
zat
zat
[43] othershell: cat junk |sort -k1,1 |tr 'c' 'z' > junk
[44] othershell: cat junk

Both environments run BASH.

I am asking this because sometimes after I do text manipulation, because of this caveat, I am forced to make the tmp file. But I know in Perl, you can give “i” flag to overwrite the original file after some operations/manipulations. I just want to ask if there is any foolproof method in unix pipeline to overwrite the file that I am not aware of.

Answer 1

In general this can be expected to break. The processes in a pipeline are all started up in parallel, so the > junk at the end of the line will usually truncate your input file before the process at the head of the pipelining has finished (or even started) reading from it.

Even if bash under Cygwin let’s you get away with this you shouldn’t rely on it. The general solution is to redirect to a temporary file and then rename it when the pipeline is complete.