bash - Correct syntax and usage of `cat` command?

Question

(This question is a follow-up on this comment, in an answer about git hooks)

I'm far too unskilled in bash (so far) to understand fully the remark and how to act accordingly. More specifically, I've been advised to avoid using bash command cat this way :

echo "$current_branch" $(cat "$1") > "$1"

because the order of operations depends on the specific shell and it could end up destroying the contents of the passed argument, so the commit message itself if I got it right?

Also, how to "save the contents in a separate step"?

Would the following make any sense?

tmp = "$1"
echo "$current_branch" $(cat $tmp) > "$1"

Answer 1

The proposed issue is not about overwriting variables or arguments, but about the fact that both reading from and writing to a file at the same time is generally a bad idea.

For example, this command may look like it will just write a file to itself, but instead it truncates it:

cat myfile > myfile   # Truncates the file to size 0

However, this is not a problem in your specific command. It is guaranteed to work in a POSIX compliant shell because the order of operations specify that redirections will happen after expansions:

2. The words that are not variable assignments or redirections shall be expanded. If any fields remain following their expansion, the first field shall be considered the command name and remaining fields are the arguments for the command.

3. Redirections shall be performed as described in Redirection.

Double-however, it's still a bit fragile in the sense that seemingly harmless modifications may trigger the problem, such as if you wanted to run sed on the result. Since the redirection (> "$1") and command substitution $(cat "$1") are now in separate commands, the POSIX definition no longer saves you:

# Command may now randomly result in the original message being deleted
echo "$current_branch $(cat "$1")" | sed -e 's/(c)/?/g' > "$1"

Similarly, if you refactor it into a function, it will also suddenly stop working:

# Command will now always delete the original message
modify_message() {
  echo "$current_branch $(cat "$1")"
}
modify_message "$1" > "$1"

You can avoid this by writing to a temporary file, and then replace your original.

tmp=$(mktemp) || exit
echo "$current_branch $(cat "$1")" > "$tmp"
mv "$tmp" "$1"