How to do `head` and `tail` on null-delimited input in bash?

Question

find command can output names of files as a null-delimited strings (if -print0 is provided), and xargs can consume them with -0 option turned on. But in between, it's hard to manipulate that collection of files - sort command has -z switch, that makes it possible to sort those files, but head and tail don't have them.

How can I do head and tail on those null-delimited inputs in a convenient way? (I can always create a short & slow ruby script, but I hope that there could be a better way)

Answer 1

GNU head and tail since coreutils version 8.25 have a -z option for that.

With older versions or for non-GNU systems, you can try and swap \0 and \n:

find ... -print0 |
  tr '\0\n' '\n\0' |
  head |
  tr '\0\n' '\n\0'

Note that some head implementations can't cope with NUL characters (and they're not required to by POSIX), but where find supports -print0, head and text utilities generally support NUL characters.

You can also use a function to wrap any command between the two trs:

nul_terminated() {
  tr '\0\n' '\n\0' | "$@" | tr '\0\n' '\n\0'
}

find ... -print0 | nul_terminated tail -n 12 | xargs -r0 ...

Keep in mind that under nul_terminated, a \0 means a newline character. So for instance, to replace \n with _:

find . -depth -name $'*\n*' -print0 | nul_terminated sed '
  p;h;s,.*/,,;s/\x0/_/g;H;g;s,[^/]*\n,,' | xargs -r0n2 mv

(\x0 being also a GNU extension).

If you need to run more than one filtering command, you can do:

find ... -print0 |
  nul_terminated cmd1 |
  nul_terminated cmd2 | xargs -r0 ...

But that means running a few redundant tr commands. Alternatively, you can run:

find ... -print0 | nul_terminated eval 'cmd1 | cmd2' | xargs -r0 ...

Answer 2

On an older system of mine, grep supported null delimited input, so I did this as a replacement for head --zero-terminated -n 3 to obtain the 3 newest files:

find . -maxdepth 1 -type f -printf "%T@ %p\0" | sort -zrn | grep -zm 3 ""

A combination of sort and grep will then allow a replacement for tail to obtain the 3rd newest file:

find . -maxdepth 1 -type f -printf "%T@ %p\0" | sort -zrn | grep -zm 3 "" | sort -zn | grep -zm 1 ""

Filename only:

find . -maxdepth 1 -type f -printf "%T@ %p\0" | sort -zrn | grep -zm 3 "" | sort -zn | grep -zPom 1 "(?s)/.*"

Sadly -o (only-matching) disables the zero delimited output (bug!?), which was enabled through -z and returns the filename with an ending new line, which disallows further piping

find . -maxdepth 1 -type f -printf "%T@ %p\0" | sort -zrn | grep -zm 3 "" | sort -zn | grep -zPom 1 "(?s)/.*" | xargs -0 ls -l
ls: cannot access /te
st
: No such file or directory

This can be solved by using read:

find . -mindepth 1 -maxdepth 1 -type f -printf "%T@ %p\0" | sort -zrn | grep -zm 3 "" | sort -zn | { IFS=" " read -rd '' mtime file && echo -en "$file\0"; } | xargs -0 -n1 ls -l
-rw-r--r-- 1 root root 0 Apr 24 17:46 ./te?st

(ls -t returns a question mark for the new line character in the filename as expected)

Or another example how to obtain the 100 oldest files in the current dir:

find /tmp/test -mindepth 1 -maxdepth 1 -type f -printf "%T@ %p\0" | sort -zn | grep -zm 10 "" | { while IFS=" " read -rd '' mtime file; do echo -en "$file\0"; done; } | xargs -0 -n1 ls -bl
-rw-r--r-- 1 root root 0 Mar 23 14:32 /tmp/test/foo\ baz.txt
-rw-r--r-- 1 root root 0 Apr 26 10:43 /tmp/test/foo\nbar\ baz.txt
...