linux - for loop without List in the shell script

Question

I am reading a shell script( /opt/openfoam8/bin/foamCleanPath) from OpenFoam. In this script, there is a code segment:

for dir
do
....
done

I marked the code segment in the following screenshot.
But I am wondering why it is NOT like for dir in someList, and there is no definition of dir in the above code segment. How should I understand it?

Here is the source code:

#!/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

To quote the Bash man page, one of the more common shells implementing /bin/sh:

for name [ [ in [ word ... ] ] ; ] do list ; done
The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once for each positional parameter that is set [...]

Thus, if you omit the in ... part, the loop will iterate over the positional arguments ($1, $2, ...) which are set in the current context. In your case, this would be the list of wildcards given as command line arguments.