Listing 1: The dateorder script reports files named as month, year (mmmyy) in chronological order.

A. Listing of the dateorder Perl program:

1 #! /usr/bin/perl 
2 # @(#) dateorder Return mmmyy command-line args in yr, month order.
3 # Author: Robert Nicholson <robert@steffi.demon.co.uk>, April 1994
4 # Usage: dateorder mmmyr-string ...
5 
6 # Given the following command-line argument list:
7 # INDEX apr90 jan89 dec90 apr89 jan90 dec89
8 # Dateorder returns:
9 # jan89 apr89 dec89 jan90 apr90 dec90
10 
11 die "Usage: $0 mmmyr-string [mmmyr-string...]\n" unless @ARGV;
12 %monthorder =
13 ("jan", 1, "feb", 2, "mar", 3, "apr", 4, "may", 5, "jun", 6,
14 "jul", 7, "aug", 8, "sep", 9, "oct", 10, "nov", 11, "dec", 12 );
15 
16 # Only collect arguments with mmmyy (month-year) prefix:
17 $i = 0; # initialize
18 foreach $file (@ARGV) {
19 if (&month_and_year_prefix($file)) { # if proper prefix
20 $monthfiles[$i++]=$file; # save argument
21 }
22 }
23 
24 @sorted_months = sort by_date @monthfiles; # order by year, month
25 print "@sorted_months\n"; # space-separated display
26 
27 # Comparator to sort by year and then by month in that year:
28 sub by_date {
29 # First three characters are the month string:
30 $amon = substr($a, 0, 3); # mmm saved in amon
31 $bmon = substr($b, 0, 3); # mmm saved in bmon
32 # The next two are the year:
33 $ayr = substr($a, 3, 2); # yy saved in ayr
34 $byr = substr($b, 3, 2); # yy saved in byr
35 
36 # Lookup index with lower-case string:
37 $amonthindex = $monthorder{"\L$amon"}; # 1-12 in amonthindex
38 $bmonthindex = $monthorder{"\L$bmon"}; # 1-12 in bmonthindex
39 
40 # Catenate the year and adjusted month index:
41 $aall = $ayr.($amonthindex+11); # create four-digit integer
42 $ball = $byr.($bmonthindex+11); # for easy comparision
43 $aall <=> $ball; # by "spaceship" operator
44 }
45 
46 # Determine if argument is mmmyy (month and year prefix):
47 sub month_and_year_prefix {
48 local($filename) = @_;
49 
50 return 0 if (length($filename) < 5);# files with names too short
51 
52 $month = substr($filename,0,3); # save month prefix
53 $year = substr($filename,3,2); # followed by the year
54 
55 foreach $key (keys %monthorder) { # for each valid month
56 return 1 if (-f $filename # only consider plain files
57 && $monthorder{$month} # with valid month abbreviation
58 && ($year =~ /\d\d/)) # and two digits for the year
59 }
60 return 0; # not the correct prefix
61 }

B. Sample usage:

$ echo *
dec91.Z index91.Z jan91.Z jul91.Z mar91.Z may91.Z nov91.Z sep91.Z
$ dateorder *
jan91.Z mar91.Z may91.Z jul91.Z sep91.Z nov91.Z dec91.Z
$ {}

C. A short simulation using conventional Unix tools, which isn't portable (see text for discussion):

$ cd /MirrorFiles/ftp.uu.net/published/unix-world/grabbag
$ ls -l 1*/* | sort +0.47 -0.51 +0.52M
-rw-r--r-- 1 bne 10451 Feb 20 1994 1990/nov90.Z
-rw-r--r-- 1 bne 4100 Feb 20 1994 1994/jan94
-rw-r--r-- 1 bne 9566 Feb 20 1994 1994/feb94
-rw-r--r-- 1 bne 5171 Mar 13 1994 1994/mar94
-rw-r--r-- 1 bne 8271 Apr 6 01:48 1994/apr94
-rw-r--r-- 1 bne 7395 May 24 01:28 1994/may94
-rw-r--r-- 1 bne 16664 May 24 01:28 1994/jun94
-rw-r--r-- 1 bne 4043 Jul 27 01:23 1994/jul94
-rw-r--r-- 1 bne 5429 Jul 30 13:24 1994/aug94
$ {}

D. Another simulation command line:

$ ls -l 1*/** | \
egrep -i '(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)[0-9][0-9]'| \
sort +0.54n +0.52M

Listing 2: Definitions of the directory-climbing functions.

A. Listing of the contributed up() function:

1 # @(#) up Move up directory hierarchy specified number of levels
2 # Author: David Wood, March 1990
3 #
4 up()
5 {
6 count=${1:-1} # default one level, otherwise use argument
7 while [ $count -gt 0 ]; do # while need to climb
8 cd .. # go up a level
9 count=`expr $count - 1` # one less level to go
10 done
11 }

B. Listing of the version ported to the Korn shell:

1 # @(#) up Move up a directory tree specified number of levels
2 # Author: David Wood, March 1990
3 # Adapted for Korn shell by Becca Thomas, December 1992
4 #
5 function up
6 {
7 Usage="Usage: `basename $0` [level-count]\\nWhere, level-count >0"
8 case $# in
9 0|1) ;; # correct argument count
10 *) print -u2 $Usage; return ;; # no more than one argument
11 esac
12 integer count=${1:-1} # default one level, otherwise use argument
13 if (( $count <= 0 )); then # if argument is not positive
14 print -u2 "level-count must be positive integer\n$Usage";return
15 fi
16 while (( $count -gt 0 )); do # while need to climb
17 cd .. # go up a level
18 let count=count-1 # one less level to go
19 done
20 }