Using diff for two files and send by email [closed] - linux

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I have files like below. I use crontab every 5 min to check the files to see if the system's added one file, for example like this: AIR_2015xxxxT0yyyyyyyy.cfg. Then I need to use the diff command automatically between the last one and before the last one.
AIR_20151021T163514000.cfg
AIR_20151026T103845000.cfg
AIR_2015xxxxT0yyyyyyyy.cfg
I want to do this in a script like the one below:
#!/bin/bash
/var/opt/fds/
diff AIR_2015xxxxT0yyyyyyyy.cfg AIR_20151026T103845000.cfg > Test.txt
body(){
cat body.txt
}
(echo -e "$(body)") | -a Test.txt mailx -s 'Comparison' user#email.com

Answer 1

Given a list of files in the directory /var/opt/fds with names in the format:
AIR_YYYYmmddTHHMMSSfff.cfg
where the letter Y represents digits for the year, m for month, d for day, H for hour, M for minute, S for second, and f for fraction (milliseconds), then you need to establish the two most recent files in the directory to compare them.
One way to do this is:
cd /var/opt/fds || exit 1
old=
new=
for file in AIR_20[0-9][0-9]????T?????????.cfg
do
old=$new
new=$file
done
if [ -n "$old" ] && [ -n "$new" ]
then
diff "$old" "$new" > test.txt
mailx -a test.txt -s 'Comparison' user#example.com < body.txt
fi
Note that if the new file has a name containing letters x and y as shown in the question and comments, it will be listed after the names containing the time stamp as digits, so it will be picked up as the new file. It also assumes permission to write in the /var/opt/fds directory, and that the mail body file is present in that directory too. Those assumptions can be trivially fixed if necessary. The test.txt file should be deleted after it is sent, too, and you could check that it is non-empty before sending the email (just in case the two most recent files are in fact identical). You could embed a time-stamp in the generated file name containing the diffs instead of using test.txt:
output="diff.$(date +'%Y%m%dT%H%M%S000').txt"
and then use $output in place of test.txt.
The test ensures that there was both an old and a new name. The pattern match is sloppier than it could be, but using [0-9] or an appropriate subrange ([01], [0-3], [0-2], [0-5]) for the question marks makes the pattern unreadably long:
for file in AIR_20[0-9][0-9][01][0-9][0-3][0-9]T[0-2][0-9][0-5][0-9][0-5][0-9][0-9][0-9][0-9].cfg
It also probably provides very little extra in the way of protection. Of course, as shown, it imposes a Y2.1K crisis on the system, not that it is hard to fix that. You could also cut down the range of valid dates by basing it on today's date, but beware of the end of the year, etc. You might decide you only need entries from the last month or so.
Using globbing is generally better than trying to parse ls or find output. In this context, where the file names have a restricted set of characters in the name (no newlines, no blanks or tabs, no quotes, no dollar signs, etc), it is feasible to use either find or ls — but if you have to deal with arbitrary names created by random end users, those tools are not suitable. (The ls command does all sorts of weird stuff with weird names and basically is hard to use reliably in the face of user cussedness. The find command and its -print0 option can be used, especially if you have a sort that recognizes -z to work with null-terminated 'lines' and an xargs that supports -0 to handle such lines too — but you have to very careful.)
Note that this scheme does not keep a record of the last file analyzed (so if no new files appear for an hour, you might send a dozen copies of the same differences), nor does it directly report on the file names (but using diff -u or diff -c would include the file names being diffed in the output). Again, these issues can be worked around if that's appropriate (and it probably is). Keeping the record of which files have been compared is probably the hardest job; even that's not too bad:
echo "$old" "$new" >> reported.diffs
to record what's been processed; then
if grep -q "$old $new" reported.diffs
then : Already processed
else : Process $old and $new
fi