Prevent "echo" from interpreting backslash escapes

Question

I'd like to echo something to a file that contains new line escape sequences, however I would like them to remain escaped. I'm looking for basically the opposite to this question.

echo "part1\npart2" >> file

I would like to look like this in the file

$ cat file
old
part1\npart2

but it looks like

$ cat file
old
part1
part2

Answer 1

This is a good example of why POSIX recommends using printf instead of echo (see here, under "application usage"): you don't know what you get with echo¹.

You could get:

• A shell builtin echo that does not interpret backslash escapes by default
  • Example: the Bash builtin echo has an -e option to enable backslash escape interpretation and checks the xpg_echo shell option
• A shell builtin echo that interprets backslash escapes by default
  • Examples: zsh, dash
• A standalone executable /bin/echo: probably depends on which one – GNU Coreutils echo understands the -e option, like the Bash builtin

The POSIX spec says this (emphasis mine):

The following operands shall be supported:
string
A string to be written to standard output. If the first operand is -n, or if any of the operands contain a <backslash> character, the results are implementation-defined.

So, for a portable solution, we can use printf:

printf '%s\n' 'part1\npart2' >> file

where the \n in the format string will always be interpreted, and the \n in the argument will never be interpreted, resulting in

part1\npart2

being appended to file.

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¹ For an exhaustive overview of various behaviours for echo and printf, see echo(1) and printf(1) on in-ulm.de.