Bash 变量替换与目录名和基名

Question

下一个脚本

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

产生:

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

问题是:

• 在 bash 脚本中，何时建议使用命令 dirname 和 basename 以及何时进行变量替换以及为什么？

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

在我的系统上产生:

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( fork )会花费时间。问题的要点是:

• 使用变量替换而不是外部命令是否存在一些陷阱？

Answer 1

外部命令进行了一些逻辑修正。检查下一个脚本的结果:

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

最有趣的结果之一是 $(dirname "aaa")。外部命令 dirname 正确返回 . 但变量扩展 ${str%/*} 返回错误值 aaa.

另一种呈现方式

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/]]