mjhjmtu

cat package.yaml > package.yaml

The above will truncate package.yaml, and hence the file will be empty - cat will have empty output.

How can I avoid truncating the file? Is there an alternative to using a temporary file? A variable maybe?

All utilities that allows for in-place editing of files does so by internally writing the result to a temporary file and then replacing the original file by this temporary file once the operation is complete.

Files can be modified in place, but you would only be able to overwrite existing data and/or extend the length of the file. This may be done using the dd utility, for example:

$ cat file.txt
hello world
abc abc 123 123

$ cat insert.txt
hello!

$ dd if=insert.txt of=file.txt bs=1 seek=6 conv=notrunc
7+0 records in
7+0 records out 
7 bytes transferred in 0.000 secs (30918 bytes/sec)

$ cat file.txt
hello hello!
bc abc 123 123

Here, we insert what's in insert.txt into file.txt by first seeking 6 bytes forward in the file (stepping past hello and the space) and then modifying it. The conv=notrunc stops the output file from being truncated at the end of the writing operation.

If if=insert.txt was left out, it would have been possible to insert any text from the keyboard. Note that "inserting" is the wrong term here really. "Overwriting" may better describe what's happening (see the first character of the second line being overwritten by the newline at the end of insert.txt).

You wouldn't want to edit a file this way though.