How to escape a NULL byte as an argument to a shel

问题:

Inside a Makefile I run a shell command which I want to pass a NULL byte as argument. The following attempt fails:

echo $(shell /bin/echo -n $$'\x00' | ruby -e "puts STDIN.read.inspect")

It generates:

echo "$\\x00"

Instead I expected:

echo "\u0000"

How do I properly escape such a NULL byte?

回答1:

echo disables interpretation of backslash escapes by default. You need to supply the -e option to enable it.

$ echo -ne "\x00" | ruby -e "puts STDIN.read.inspect"
"\u0000"

回答2:

Due to the execve(2) semantics it is not possible to pass a string containing a null byte as argument. Each argument string is terminated by null byte, therefore making it impossible to distinguish between the contained null byte and the end of the string.

回答3:

These uses of echo are totally non-portable. Use printf, it's much easier to use for anything other than the simplest strings, and much more portable.

$ cat makefile
all:
        printf '\0' > foo.out
        od -a foo.out

$ make
printf '\0' > foo.out
od -a foo.out
0000000 nul
0000001

回答4:

If anyone else came here looking how to escape a null via a shell command in ruby backticks:

irb(main):024:0> `curl --silent http://some-website-or-stream.com | sed 's/\\x0//g' 1>&2`
=> ""