Need explanation of script


 
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Old 11-25-2007
Need explanation of script

Hi
All,


Can anybody explain what this script is doing?

#!/bin/sh
who | cut -d " " -f1 | sort -u > userlist1
while true ; do
sleep 60
who | cut -d" " -f1 | sort -u >userlist2
for username in `cat userlist1` ; do
if ! grep "^$username$" userlist2 > /dev/null ; then
echo "$username has logged out"
fi
done
for username in `cat userlist2` ; do
if ! grep "^$username$" userlist1>/dev/null; then
echo "$username hass logged in"
done
mv userlist2 userlist1;
done

Thanks,
Vishal
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CUT(1)							      General Commands Manual							    CUT(1)

NAME
cut - select out columns of a file SYNOPSIS
cut [ -b | -c] list [file...] cut -f list [-d delim] [ -s] OPTIONS
-b Cut specified bytes -c Select out specific characters -d Change the column delimiter to delim -f Select out specific fields that are separated by the -i Runs of delimiters count as one -s Suppres lines with no delimiter characters, when used EXAMPLES
cut -f 2 file # Extract field 2 cut -c 1-2,5 file # Extract character columns 1, 2, and 5 cut -c 1-5,7- file # Extract all columns except 6 DESCRIPTION
[file...]" delimiter character ( see delim)" with the -f option. Lines with no delimiters are passwd through untouched" Cut extracts one or more fields or columns from a file and writes them on standard output. If the -f flag is used, the fields are sepa- rated by a delimiter character, normally a tab, but can be changed using the -d flag. If the -c flag is used, specific columns can be specified. The list can be comma or BLANK separated. The -f and -c flags are mutually exclusive. Note: The POSIX1003.2 standard requires the option -b to cut out specific bytes in a file. It is intended for systems with multi byte characters (e.g. kanji), since MINIX uses only one byte characters, this option is equivalent to -c. For the same reason, the option -n has no effect and is not listed in this man- ual page. SEE ALSO
sed(1), awk(9). CUT(1)