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linuxbashsedxargs

Random behaviour when pipping find output to xargs and then to sed

I'm using the gnu version of these tools. I'm trying to unzip an archive and transform a file.

The file is "myfile.txt" and it appears in multiple folders in the archive- so I thought passing the full path to xarg would transform all files:

mkdir temp
unzip mypackage.zip -d temp

find temp -iname "myfile.txt" | xargs -I FILE sh -c "sed -e 's/replacethis/withthis/g' -e 's/replacethistoo/withthisaswell/g' FILE | tee FILE"
# List the files
find temp -iname "myfile.txt" | xargs -I FILE ls -l FILE
# Cat the files
find temp -iname "myfile.txt" | xargs -I FILE cat FILE
# Clean up 
rm -Rf temp

I run this script multiple times and have different outcomes which I don't understand.

Each time a different "myfile.txt" is modified, sometimes one of the "myfile.txt" files has 0 bytes

Why is this happening? It should be the same every time, shouldn't it? Is find only passing one, random, "myfile.txt" path to xargs each time I run this script?

Solution

  • Why is this happening? It should be the same every time shouldnt it?

    This happens because of a race condition between the two parallel operations of:

    • sed opening and reading the file
    • tee opening and truncating the file

    If tee wins, the file will be empty when sed reads it, and it will therefore be 0 bytes.

    If sed wins, it'll read (at least parts of) the file and you'll get some data.

    Since process scheduling is not predictable, you risk seeing different results each time.