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UNIX tools, spaces in filenames and \0


A normal way to delete all files containing _old in the current folder (on a UNIX system) is a command like the following:

find . -iname '*_old*' | xargs rm

However this doesn’t work for filenames containing spaces as rm expects spaces to separate each file.

The usual solution to this is to add -print0 to the find command and -0 to xargs:

find . -iname '*_old*' -print0 | xargs -0 rm

Now my problem is that I only want to delete the first 5 files. The “UNIX way” is to do something like:

find . -iname '*_old*' | head -n 5 | xargs rm

– and now we’re back to something that doesn’t work with filenames containing spaces. Our -print0-trick from earlier won’t work because head doesn’t have an equivalent of the xargs -0 flag.

awk to the rescue?

Note: The following assumes that awk refers to BSD-awk – specifically the awk provided on OS X 10.7.5.

awk operates on a line-by-line basis by default, but this can be changed with the record separator variable RS. A simple head -n 5 implementation would be awk 'NR <= 5 {print}' in awk.

Lets say we’re in a folder with these files:

> ls -1
Los Angeles vacation_001_old.jpg
Los Angeles vacation_002_old.jpg
Los Angeles vacation_003_old.jpg
Los Angeles vacation_004_old.jpg
Los Angeles vacation_005_old.jpg
Los Angeles vacation_006_old.jpg
Los Angeles vacation_007_old.jpg
Los Angeles vacation_008_old.jpg
Los Angeles vacation_009_old.jpg

With awk we should be able to do something like this:

> find . -iname '*_old*' -print0 | awk 'BEGIN {RS="\0"}; NR <= 5 {print}'
./Los Angeles vacation_001_old.jpg

… but it doesn’t work: awk never finds records beyond the first one because of the NUL character \0. The NUL character is the C-string terminator and thus AWK stops reading input after seeing it because it thinks the input string has ended.

However the more modern awks gawk and mawk doesn’t suffer from the same shortcoming:

> find . -iname '*_old*' -print0 | mawk 'BEGIN {RS="\0"}; NR<=5 {print}'
./Los Angeles vacation_001_old.jpg
./Los Angeles vacation_002_old.jpg
./Los Angeles vacation_003_old.jpg
./Los Angeles vacation_004_old.jpg
./Los Angeles vacation_005_old.jpg

But the print function in awk (+ mawk and gawk) print newlines by default. We can change this by setting the output field separator ORS

> find . -iname '*_old*' -print0 | mawk 'BEGIN {RS=ORS="\0"}; NR<=5 {print}'
./Los Angeles vacation_001_old.jpg./Los Angeles vacation_002_old.jpg./Los Angeles vacation_003_old.jpg./Los Angeles vacation_004_old.jpg./Los Angeles vacation_005_old.jpg

The NUL characters doesn’t print – but they are there – you can check by piping into cat -v.

Finally we reach the command:

> find . -iname '*_old*' -print0 | mawk 'BEGIN {RS=ORS="\0"}; NR<=5 {print}' | xargs -0 rm

In sed you can do something like sed -n '1,5p' but sed expects newlines and has no setting to change the “separator” to be something other than newline (sed is Turing-complete, however, so nothing is impossible1).


I think the UNIX tools are great. No doubt about it. I use them every day, and the “do one thing, and do it well”-philosophy plus the composability of tools through pipes are some of the things that these tools invaluable to many programmers, including me. BUT the tools are from the 80’s (and sometimes even older) and have fallen out of touch with modern computing. Today the users’ needs and behaviour shapes the systems rather than the other way around.

We need to build tools that can handle filenames containing spaces and unicode characters. I have an older blog post that touches the same topic: The Unix shebang (#!).

For now I’ve made a head “implementation” for input separated by NULs instead of newlines:

head0() {
  mawk 'BEGIN {RS="\0"}; {print}' | head $@ | mawk 'BEGIN {ORS="\0"}; {print}'

Should we replace the UNIX tools with more modern, but equivalent tools? Is a command-line like PowerShell with built-in types the way to go?

One thing is clear to me: a solution to these problems need to be found. And preferably one that fixes them once and for all.