在带有 if 条件的 for 循环中强制按字母顺序排列

Question 1

#/bin/sh

# shellcheck disable=SC2046
# ^ word-splitting by the shell is intentional in this file

elems="Cr Hf Mo Nb Ta Ti V W Zr"
for a in $elems
do
    for b in $elems
    do
        for c in $elems
        do
            for d in $elems
            do
                # for a set of any four elements:
                #   string them together, separated by NUL-bytes
                #   sort them lexicographically ...
                #     ... with NUL separating the elements (-z)
                #     ... and eliminate duplicates (-u)
                #   then replace the NUL bytes with line breaks
                #   allow the shell to split on those line breaks
                #   and chuck the resulting chunks into $1, $2, etc
                set -- $(printf '%s\0' "$a" "$b" "$c" "$d" | sort -z -u | tr "\0" "\n")

                # only if the current selection of elements consisted of four
                # different ones (remember we eliminated duplicates):
                if [ $# -eq 4 ]
                then
                    # create a directory, don't error out if it already exists (-p)
                    mkdir -p "$(printf '%s' "$@")"
                fi
            done
        done
    done
done

效率不高（sort甚至调用明显的非候选者和多次mkdir调用同一目录名），但内部循环最多 9 ⁴ = 6561 次迭代，而且它是一次性脚本，我认为不会这是值得花费大量时间进行优化的。

编辑：
Xeon E3-1231v3 的基准测试，没有mkdir：

./elemdirs.sh > /dev/null  11.66s user 1.73s system 173% cpu 7.725 total

并随之而来：

./elemdirs.sh > /dev/null  13.80s user 2.16s system 156% cpu 10.215 total

它产生 126 个目录，预期数量组合其中 k = 4，n = 9。

Answer

#/bin/sh

# shellcheck disable=SC2046
# ^ word-splitting by the shell is intentional in this file

elems="Cr Hf Mo Nb Ta Ti V W Zr"
for a in $elems
do
    for b in $elems
    do
        for c in $elems
        do
            for d in $elems
            do
                # for a set of any four elements:
                #   string them together, separated by NUL-bytes
                #   sort them lexicographically ...
                #     ... with NUL separating the elements (-z)
                #     ... and eliminate duplicates (-u)
                #   then replace the NUL bytes with line breaks
                #   allow the shell to split on those line breaks
                #   and chuck the resulting chunks into $1, $2, etc
                set -- $(printf '%s\0' "$a" "$b" "$c" "$d" | sort -z -u | tr "\0" "\n")

                # only if the current selection of elements consisted of four
                # different ones (remember we eliminated duplicates):
                if [ $# -eq 4 ]
                then
                    # create a directory, don't error out if it already exists (-p)
                    mkdir -p "$(printf '%s' "$@")"
                fi
            done
        done
    done
done

效率不高（sort甚至调用明显的非候选者和多次mkdir调用同一目录名），但内部循环最多 9 ⁴ = 6561 次迭代，而且它是一次性脚本，我认为不会这是值得花费大量时间进行优化的。

编辑：
Xeon E3-1231v3 的基准测试，没有mkdir：

./elemdirs.sh > /dev/null  11.66s user 1.73s system 173% cpu 7.725 total

并随之而来：

./elemdirs.sh > /dev/null  13.80s user 2.16s system 156% cpu 10.215 total

它产生 126 个目录，预期数量组合其中 k = 4，n = 9。

Question 2

使用 Perl 和Algorithm::Combinatorics模块：

perl -MAlgorithm::Combinatorics=combinations -e '$"=""; map { mkdir "@{$_}N" } combinations([qw(Cr Hf Mo Nb Ta Ti V W Zr)], 4)'

这将创建 126 个目录，您可以从包含的四个单词的所有组合中获得这些目录。每个目录的名称N末尾都有一个。由于代码中数组的初始排序，各个单词将始终按字母顺序出现在目录名称中。

作为一个正确的 Perl 脚本：

#!/usr/bin/perl

use strict;
use warnings;

use English;
use Algorithm::Combinatorics qw(combinations);

# When interpolating a list in a string (@{$ARG} below), don't use a delimiter
local $LIST_SEPARATOR = "";

# Get all combinations, and create a directory for each combination
map { mkdir "@{$ARG}N" } combinations( [qw(Cr Hf Mo Nb Ta Ti V W Zr)], 4 );

这几乎可以立即运行，并且可以轻松扩展以包含更多单词或组合长度。

你可能可以在 Python 中做一些非常类似的事情......

递归 shell 实现（只是为了好玩，递归 shell 函数很少非常有效）：

#!/bin/sh

build_combinations () {
    set_size=$1
    shift

    if [ "$set_size" -eq 0 ]; then
        printf 'N'
    else
        for token do
            shift
            for reminder in $(build_combinations "$(( set_size - 1 ))" "$@")
            do
                printf '%s%s\n' "$token" "$reminder"
            done
        done
    fi
}

build_combinations 4 Cr Hf Mo Nb Ta Ti V W Zr | xargs mkdir

读过的想法斯图狗的回答以及来自各个方面的灵感StackOverflow 问题的答案。

请注意，此解决方案的优点是目录名称始终以N.递归停止分支输出N而不是空字符串，这使得整个事情正常进行。如果没有它（打印空字符串或换行符），带有命令替换的循环将没有任何内容可循环，并且不会有输出（由于变量的默认值IFS）。

Answer