noa.md

Motivation

Long ago Unix (in getopt) and later POSIX standardized upon a "--" token as a separator between command options and ordinary arguments. However, many commands on Unix (and elsewhere!) have behavior sensitive only to the location of ordinary arguments. E.g., the final argument of cp & mv behave very differently from all the others. Together these two features complicate writing reliable scripts that wrap such commands to, say, create a target directory only if it does not already exist.

This is where this simple program called noa comes in to simplify life.¹ noa extracts command parameters by index over only the non-option arguments. Here indices are signed like Python with <0 indicating "backwards from the end". Some programs (to be more xargs friendly, say) may take similar dependencies like cp & mv but at index 0. But noa is a general non-option indexer.

Usage

(n)on-(o)ption (a)rgument usage:

  noa {index} options-and-args

E.g.: noa -1 cp -a foo -f -- /exists/maybe/missing
emits "/exists/maybe/missing" no matter where "--" is.
Can be nice in scripts to e.g. ensure must-haves exist.

Note that noa -1 ... even works with a trailing non-argument "--" separator.

Example Script

One example application of this non-option argument indexing idea is a simple but somewhat general meta-wrapper script below named ea. This makes it easy to issue

ea mv -v -n -- foo /exists/maybe/missing

and have it work even if /exists/maybe/missing does not exist yet with just a few extra keystrokes. Here is a somewhat careful/complete ea in POSIX shell in terms of noa:

#!/bin/sh
set -e
: "${idx:=-1}"              # final non-option parameter targets are frequent
: "${make:=mkdir -p --}"    # mkdir -p is often an ok idea for missing targets
if [ $# -lt 1 ]; then
    cat <<EOF
E)nsure A)rgument Usage:
  [idx=-1] [make="mkdir -p --"] [v=] ea {cmd needing an argument to exist}
where
  \$idx  is a Python like 0-origin or length-relative negative index
  \$make is a prefix to a command to create the argument if missing
  \$v    set to anything means echo \$make before running it
E.g.:
  ea cp -a foo -f -- /exists/maybe/missing
  v= ea mv -v -n -- foo /exists/maybe/missing
EOF
    exit 1
fi
if ! type noa >/dev/null 2>&1; then
    echo 1>&2 'Need to install `noa` from https://github.com/c-blake/bu'
    exit 2
fi
noa "$idx" "$@" | (         # Carefully d=`noa $idx $*`; [ -e $d ] || $make $d
    n='
'                           # read -rd works in Bash|Zsh, but not POSIX.  So,
    while IFS= read -r line #..loop which works for all text but input with no
    do d="$d${line}$n"      #..final newline where we add one "erroneously".
    done; d=${d%?}          # Chop extra newline
    [ -e "$d" ] || {        # not -d since some cmds have -f to replace files
        [ -n "${v+ANY}" ] && printf '%s\n' "$make $d"
        $make "$d"          # Make needed argument $d
    } )
exec "$@"                   # Then just run passed command

Footnotes

1. It could likely be written reliably in pure shell (have at it!), but noa is also very simple in lower level languages (even as low as ANSI C). ↩