在 shell 脚本中没有 List 的 for 循环

Question

我正在阅读来自OpenFoam 的shell 脚本( /opt/openfoam8/bin/foamCleanPath)。在这个脚本中，有一段代码：

for dir
do
....
done

我在以下屏幕截图中标记了代码段。
但是我想知道为什么它不像for dir in someList，并且上面的代码段中没有dir的定义。我应该怎么理解？

这里是源代码：

#!/bin/sh
#------------------------------------------------------------------------------
# =========                 |
# \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
#  \\    /   O peration     | Website:  https://openfoam.org
#   \\  /    A nd           | Copyright (C) 2011-2018 OpenFOAM Foundation
#    \\/     M anipulation  |
#------------------------------------------------------------------------------
# License
#     This file is part of OpenFOAM.
#
#     OpenFOAM is free software: you can redistribute it and/or modify it
#     under the terms of the GNU General Public License as published by
#     the Free Software Foundation, either version 3 of the License, or
#     (at your option) any later version.
#
#     OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
#     ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
#     FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
#     for more details.
#
#     You should have received a copy of the GNU General Public License
#     along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
#
# Script
#     foamCleanPath
#
# Description
#     Usage: foamCleanPath [-strip] path [wildcard] .. [wildcard]
#
#     Prints its argument (which should be a ':' separated path)
#     without all
#         - duplicate elements
#         - elements whose start matches a wildcard
#         - inaccessible directories (with the -strip (at your option)
#
# Note
#     - this routine will fail when directories have embedded spaces
#     - false matches possible if a wildcard contains '.' (sed regex)
#     - the wildcards can themselves can be written together and separated
#       by colons or whitespace
#------------------------------------------------------------------------------
usage() {
    cat <<USAGE
Usage: ${0##*/} [OPTION] path [wildcard1] .. [wildcardN]
options:
  -strip            remove inaccessible directories
  -help             print the usage

  Prints its argument (which should be a ':' separated list) cleansed from
    - duplicate elements
    - elements whose start matches one of the wildcard(s)
    - inaccessible directories (with the -strip option)

  Exit status
      0  on success
      1  for miscellaneous errors.
      2  initial value of 'path' is empty

USAGE
}

error() {
    usage 1>&2
    exit 1
}


unset strip
# parse options
while [ "$#" -gt 0 ]
do
    case "$1" in
    -h | -help)
        usage && exit 0
        ;;
    -strip)
        strip=true
        shift
        ;;
    -*)
        error
        ;;
    *)
        break
        ;;
    esac
done


[ "$#" -ge 1 ] || error

dirList="$1"
shift

[ -n "$dirList" ] || exit 2    # quick exit on empty 'dirList'


##DEBUG echo "input>$dirList<" 1>&2

# preserve current IFS and split on colon or whitespace
oldIFS="$IFS"
IFS=': '

# "wildcard1 ... wildcardN" may have been passed as a single parameter
# or may contain ':' separators
set -- $*

# strip out wildcards via sed
while [ "$#" -ge 1 ]
do
    wildcard=$1
    shift
    ##DEBUG echo "remove>$wildcard<" 1>&2
    if [ -n "$wildcard" ]
    then
        dirList=$(echo "$dirList:" | sed -e "s|${wildcard}[^:]*:||g")
    fi
done

# split on ':' (and on space as well to avoid any surprises)
IFS=': '
set -- $dirList

##DEBUG echo "intermediate>$dirList<" 1>&2

# rebuild the list from scratch
unset dirList
for dir
do
    ##DEBUG echo "check>$dir<" 1>&2
    #- dirs must exist
    if [ -e "$dir" ]
    then
        #- no duplicate dirs
        duplicate=$(echo " $dirList " | sed -ne "s: $dir :DUP:p")

        if [ ! "$duplicate" ]
        then
            dirList="$dirList $dir"
        fi
    elif [ "$strip" != true ]
    then
        # Print non-existing directories if not in 'strip' mode.
        dirList="$dirList $dir"
    fi
done

# split on whitespace
IFS=' '
set -- $dirList

# rejoin on ':'
IFS=':'
dirList="$*"

# restore IFS
IFS="$oldIFS"

##DEBUG echo "output>$dirList<" 1>&2
echo "$dirList"

#------------------------------------------------------------------------------

Answer 1

引用Bash man page，这是实现/bin/sh 的更常见的shell 之一：

for name [ [ in [ word ... ] ] ; ] do list ; done
in 之后的单词列表被展开，生成一个项目列表。变量name依次设置到这个列表的每个元素，每次执行list。如果in word 被省略，for 命令对每个设置的位置参数执行一次list [...]

因此，如果您省略 in ... 部分，循环将遍历在当前上下文中设置的位置参数（$1、$2、...）。在您的情况下，这将是作为命令行参数给出的 通配符 列表。