Given that you have two lists of files in sorted order (since ls lists the files in sorted order), you should probably be using a command like diff or, in this case,
comm to find the differences between the two lists of files.
If you want to know which file(s) were created, then that's the list of files (lines) in the second file that are not in the first. With no options, comm lists the lines it reads in 3 columns:
- lines in the first file not in the second
- lines in the second file not in the first
- lines in both files
You only need the lines (file names) in the second column, and therefore you want to suppress the list of files in the first and third columns, so you'll use comm -13 to do that:
before=$(mktemp ${TMPDIR:-/tmp}/files.XXXXXX)
after=$(mktemp ${TMPDIR:-/tmp}/files.XXXXXX)
trap "rm -f $before $after; exit 1" 0 1 2 3 13 15
ls > $before
…execute command that creates file(s)…
ls > $after
comm -13 $before $after
rm -f $before $after
trap 0
Obviously, you could capture the list of files from comm in a variable for further analysis, etc.
Making sed work when the search strings contain metacharacters
I'm still confused about sed. How do I use a variable in the search pattern of sed if the value contains metacharacters? Or in this case would I be better off using something other than sed?
In the scenario you have, you're far better off not using sed, and in any case your technique is horrendously slow if there are hundreds or thousands of files in the directory (running sed once per file name is not going to be fast).
However, supposing that it was necessary to use sed and that you wanted to deal with metacharacters in the file names in the list, then you would have to escape the metacharacters (with a backslash in front). I'd probably do something like this:
sed 's/[][\/*.]/\\&/g; s%.*%/^&$/d%' listfilesbefore.swp > script.sed
sed -f script.sed listfilesafter.swp
The first script takes any metacharacters in the line (file name) and replaces it with backslash-metacharacter. In the first substitute, the [][\/*.] character class matches square brackets, two types of slashes, stars and dots. Depending on the predilections of the variant of sed you're using, you might need to protect (){} with backslashes too, but in POSIX standard sed, the {} gain metacharacter meaning when prefixed with a backslash, so they're not modified by default. The second substitute takes the possibly modified line and converts it into a 'match and delete' command. The output, therefore, is a sed script that will delete the file names found in listfilesbefore.swp. The second command applies that script to listfilesafter.swp, doing in one sed command what your outline code does with one run of sed per file name.
Using sed to generate a sed script is a powerful technique. It isn't always appropriate, but when it is, it is very useful.
Shell script demo.sh
echo "Pre-populate the directory with some random file names"
for file in $(random -n 20 -T '%W%V%C-%w%v%c%v%c-%04[0000:9999]d.txt')
do
cp /dev/null $file
done
for template in '%w%v%w(%03[000:999]d)%w%v%w.txt' \
'%w%v%w[123]%w%v%we.txt' \
'%w%v%wfile*%03[0:999]d*.txt' \
'%w%v%w%v%c\\\%d.txt' \
'%w%v%w-{%04X}-{%04X}.txt'
do
for file in $(random -n 2 -T "$template")
do
cp /dev/null "$file"
done
done
ls > listfilesbefore.swp
ls
echo
echo "Create some new files with metacharacters in the names"
for file in 'new(123)file.txt' 'new[123]file.txt' 'newfile*321*.txt' \
'newfile\\\.txt' 'newfile-{A39F}-{B77D}.txt'
do
cp /dev/null "$file"
done
ls
ls > listfilesafter.swp
echo
echo "Create sed script"
sed 's/[][\/*.]/\\&/g; s%.*%/^&$/d%' listfilesbefore.swp > script.sed
echo
cat script.sed
echo
echo "Apply it"
sed -f script.sed listfilesafter.swp
The random command I'm using is of my own devising, but it is convenient for demonstrations such as this.
Example run
Pre-populate the directory with some random file names
AIG-taral-3486.txt
COV-oipuc-9088.txt
CUG-vowan-5758.txt
FEH-ieqek-0603.txt
IUS-aaduw-7080.txt
KER-jazuc-4824.txt
MIZ-iezec-8255.txt
NIT-kupib-6873.txt
PUX-oocov-2216.txt
QAW-xonod-3937.txt
QES-wawok-4790.txt
RON-difag-1986.txt
SAD-gesug-5706.txt
SAJ-luqoj-4311.txt
TUZ-wapaw-8547.txt
VAL-zutap-8054.txt
YIP-xudeb-7397.txt
YUP-uudiv-8848.txt
ZIB-jurax-2903.txt
ZUR-xonik-8800.txt
aavfile*147*.txt
demo.sh
diman\\\7115.txt
ganur\\\8732.txt
gud-{7049}-{3103}.txt
listfilesbefore.swp
lur[123]maee.txt
rivfile*065*.txt
ueo(417)yea.txt
uoi(751)qio.txt
woi-{37E8}-{009C}.txt
xof[123]hoxe.txt
Create some new files with metacharacters in the names
AIG-taral-3486.txt
COV-oipuc-9088.txt
CUG-vowan-5758.txt
FEH-ieqek-0603.txt
IUS-aaduw-7080.txt
KER-jazuc-4824.txt
MIZ-iezec-8255.txt
NIT-kupib-6873.txt
PUX-oocov-2216.txt
QAW-xonod-3937.txt
QES-wawok-4790.txt
RON-difag-1986.txt
SAD-gesug-5706.txt
SAJ-luqoj-4311.txt
TUZ-wapaw-8547.txt
VAL-zutap-8054.txt
YIP-xudeb-7397.txt
YUP-uudiv-8848.txt
ZIB-jurax-2903.txt
ZUR-xonik-8800.txt
aavfile*147*.txt
demo.sh
diman\\\7115.txt
ganur\\\8732.txt
gud-{7049}-{3103}.txt
listfilesbefore.swp
lur[123]maee.txt
new(123)file.txt
new[123]file.txt
newfile*321*.txt
newfile-{A39F}-{B77D}.txt
newfile\\\.txt
rivfile*065*.txt
ueo(417)yea.txt
uoi(751)qio.txt
woi-{37E8}-{009C}.txt
xof[123]hoxe.txt
Create sed script
/^AIG-taral-3486\.txt$/d
/^COV-oipuc-9088\.txt$/d
/^CUG-vowan-5758\.txt$/d
/^FEH-ieqek-0603\.txt$/d
/^IUS-aaduw-7080\.txt$/d
/^KER-jazuc-4824\.txt$/d
/^MIZ-iezec-8255\.txt$/d
/^NIT-kupib-6873\.txt$/d
/^PUX-oocov-2216\.txt$/d
/^QAW-xonod-3937\.txt$/d
/^QES-wawok-4790\.txt$/d
/^RON-difag-1986\.txt$/d
/^SAD-gesug-5706\.txt$/d
/^SAJ-luqoj-4311\.txt$/d
/^TUZ-wapaw-8547\.txt$/d
/^VAL-zutap-8054\.txt$/d
/^YIP-xudeb-7397\.txt$/d
/^YUP-uudiv-8848\.txt$/d
/^ZIB-jurax-2903\.txt$/d
/^ZUR-xonik-8800\.txt$/d
/^aavfile\*147\*\.txt$/d
/^demo\.sh$/d
/^diman\\\\\\7115\.txt$/d
/^ganur\\\\\\8732\.txt$/d
/^gud-{7049}-{3103}\.txt$/d
/^listfilesbefore\.swp$/d
/^lur\[123\]maee\.txt$/d
/^rivfile\*065\*\.txt$/d
/^ueo(417)yea\.txt$/d
/^uoi(751)qio\.txt$/d
/^woi-{37E8}-{009C}\.txt$/d
/^xof\[123\]hoxe\.txt$/d
Apply it
listfilesafter.swp
new(123)file.txt
new[123]file.txt
newfile*321*.txt
newfile-{A39F}-{B77D}.txt
newfile\\\.txt
