CAT 忽略换行符

Question

I am a learner and I just created a new bash script that reads two filesnames from the command line and prints their content concatenated. However, it doesn't work. I have two files: log1.txt and log2.txt, both of which end with newline. My code is basically this:

 File+=$(cat "$first")
 File+=$(cat "$second")
 echo "$File"

However, the end of the first file and the beginning of the second file are merged in a single line. How do I fix this? I've been googling for hours and I will appreciate any help.

Answer 1

Command substitution trims any trailing newline in the output of the command run. This behavior is mandated by the POSIX standard -- and it would make no sense for it to be otherwise, as POSIX specifies that all valid lines of text on UNIXy systems must be terminated by a newline; so folks would be needing to trim trailing newlines constantly were it otherwise.

---

However, you have no reason to make it your shell's problem to introduce that newline:

cat stands for "concatenate"; when given multiple files, it concatenates them together. If you aren't passing it more than one argument, you typically have no good reason to use it in the first place.

File=$(cat "$first" "$second")

Answer 2

Command substitutions remove any trailing newlines (even multiple newlines) at the end of standard output from the compound command inside. The two main ways around this is to either

1. make sure the command substitution ends in some other terminator which you then remove or
2. use something other than a command substitution.

A simple implementation of the first is this pattern:

name="$(some_command; echo x)"
name="${name%x}"

The simplest way to avoid command substitution is to concatenate each file to a separate file:

cleanup() {
    if [[ -d "$work_dir" ]]
    then
        rm --force --recursive "$work_dir"
    fi
}
trap cleanup EXIT
work_dir="$(mktemp --directory)"
aggregate_file="${work_dir}/aggregate"
cat "$first" > "$aggregate_file"
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