Bash variable substitution vs dirname and basename

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

with the result

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

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.

Alternative presentation

Script:

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//

Output:

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/]]       

1. dirname outputs . if its parameter doesn't contain a slash /, so emulating dirname with parameter substitution does not yield the same results depending on the input.

2. basename takes a suffix as second parameter which will also remove this component from the filename. You can emulate this as well using parameter substitution but since you cannot do both at once it is not as brief as when using basename.

3. Using either dirname or basename require a subshell since they are not shell builtins, so the parameter substitution will be faster, especially when calling them in a loop (as you have shown).

4. I have seen basename in different locations on different systems (/usr/bin, /bin) so if you have to use absolute paths in your script for some reason it might break since it cannot find the executable.

So, yes, there are some things to consider and depending on situation and input I use both methods.

EDIT: Both dirname and basename are actually available as bash loadable builtins under examples/loadables in the source tree and can be enabled (once compiled) using

enable -f /path/to/dirname dirname
enable -f /path/to/basename basename