Bash的变量替换VS目录名称和基本名称

问题描述

接下来的剧本

str=/aaa/bbb/ccc.txt
echo "str: $str"
echo ${str##*/} == $(basename $str)
echo ${str%/*} == $(dirname $str)

生产：

str: /aaa/bbb/ccc.txt
ccc.txt == ccc.txt
/aaa/bbb == /aaa/bbb

现在的问题是：

• 在的bash脚本，如果是，则建议使用命令目录名和基本名键，当变量替换和为什么？

• In bash scripts, when is it recommended to use commands dirname and basename and when the variable substitutions and why?

问这主要是因为：

str="/aaa/bbb/ccc.txt"
count=10000

s_cmdbase() {
let i=0
while(( i++ < $count ))
do
    a=$(basename $str)
done
}

s_varbase() {
let i=0
while(( i++ < $count ))
do
    a=${str##*/}
done
}

s_cmddir() {
let i=0
while(( i++ < $count ))
do
    a=$(dirname $str)
done
}

s_vardir() {
let i=0
while(( i++ < $count ))
do
    a=${str%/*}
done
}

time s_cmdbase
echo command basename
echo ===================================
time s_varbase
echo varsub basename
echo ===================================
time s_cmddir
echo command dirname
echo ===================================
time s_vardir
echo varsub dirname

在我的系统产生的：

on my system produces:

real    0m33.455s
user    0m10.194s
sys     0m18.106s
command basename
===================================

real    0m0.246s
user    0m0.237s
sys     0m0.007s
varsub basename
===================================

real    0m30.562s
user    0m10.115s
sys     0m17.764s
command dirname
===================================

real    0m0.237s
user    0m0.226s
sys     0m0.007s
varsub dirname

调用外部程序（分叉）花费的时间。这个问题的主要观点是：

Calling external programs (forking) costs time. The main point of the question is:

• 是否有一些陷阱使用的变量替换，而不是外部命令？

推荐答案

的外部命令做出一些合理的修正。检查下一个脚本的结果是：

The external commands make some logical corrections. Check the result of the next script:

doit() {
    str=$1
    echo -e "string   $str"
    cmd=basename
    [[ "${str##*/}" == "$($cmd $str)" ]] && echo "$cmd same: ${str##*/}" || echo -e "$cmd different \${str##*/}\t>${str##*/}<\tvs command:\t>$($cmd $str)<"
    cmd=dirname
    [[ "${str%/*}"  == "$($cmd $str)" ]] && echo "$cmd  same: ${str%/*}" || echo -e "$cmd  different \${str%/*}\t>${str%/*}<\tvs command:\t>$($cmd $str)<"
    echo
}

doit /aaa/bbb/
doit /
doit /aaa
doit aaa
doit aaa/
doit aaa/xxx

，结果

string   /aaa/bbb/
basename different ${str##*/}   ><          vs command: >bbb<
dirname  different ${str%/*}    >/aaa/bbb<  vs command: >/aaa<

string   /
basename different ${str##*/}   ><  vs command: >/<
dirname  different ${str%/*}    ><  vs command: >/<

string   /aaa
basename same: aaa
dirname  different ${str%/*}    ><  vs command: >/<

string   aaa
basename same: aaa
dirname  different ${str%/*}    >aaa<   vs command: >.<

string   aaa/
basename different ${str##*/}   ><  vs command: >aaa<
dirname  different ${str%/*}    >aaa<   vs command: >.<

string   aaa/xxx
basename same: xxx
dirname  same: aaa

一个最有趣的结果是 $（目录名称AAA）。外部命令目录名正确返回。但变量扩展 $ {STR％/ *} 返回不正确的值 AAA 。

One of most interesting results is the $(dirname "aaa"). The external command dirname correctly returns . but the variable expansion ${str%/*} returns the incorrect value aaa.

脚本：

doit() {
    strings=( "[[$1]]"
    "[[$(basename "$1")]]"
    "[[${1##*/}]]"
    "[[$(dirname "$1")]]"
    "[[${1%/*}]]" )
    printf "%-15s %-15s %-15s %-15s %-15s\n" "${strings[@]}"
}


printf "%-15s %-15s %-15s %-15s %-15s\n" \
    'file' 'basename $file' '${file##*/}' 'dirname $file' '${file%/*}'

doit /aaa/bbb/
doit /
doit /aaa
doit aaa
doit aaa/
doit aaa/xxx
doit aaa//

输出：

file            basename $file  ${file##*/}     dirname $file   ${file%/*}
[[/aaa/bbb/]]   [[bbb]]         [[]]            [[/aaa]]        [[/aaa/bbb]]
[[/]]           [[/]]           [[]]            [[/]]           [[]]
[[/aaa]]        [[aaa]]         [[aaa]]         [[/]]           [[]]
[[aaa]]         [[aaa]]         [[aaa]]         [[.]]           [[aaa]]
[[aaa/]]        [[aaa]]         [[]]            [[.]]           [[aaa]]
[[aaa/xxx]]     [[xxx]]         [[xxx]]         [[aaa]]         [[aaa]]
[[aaa//]]       [[aaa]]         [[]]            [[.]]           [[aaa/]]

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