Bash: How to check for first three characters in a file - string

Question

After some string conversion of heterogeneous data, there are files with the following content:
file1.txt:
mat 445
file2.txt:
mat 734.2
and so on. But there are also intruders that do not match that pattern, e. g.
filen.txt:
mat 1
FBW
With everything that starts with "mat" I would like to proceed while all other lines shall be deleted.
The following does not work (and seems rather ponderous):
for f in *.txt ; do
if [[ ${f:0:3} == "mat" ]]; then
# do some string conversion with that line, which is not important here
sed -i -e 's/^.*\(mat.*\).*$/\1/' $f
sed -i -e 's/ //g' $f
tr '.' '_' < $f
sed -i -e 's/^/\<http:\/\/uricorn.fly\/tib\_lok\_sys\#/' "$f"
sed -i -e 's/\(.*\)[0-9]/&> /' "$f"
else
# delete the line that does not match the pattern
sed -i -e '^[mat]/d' $f
fi
done
As the comment below shows the if condition is incorrect as it does not match the file's content but its name.
Desired output should then be:
file1.txt
<http://uricorn.fly/tib_lok_sys#mat445>
file2.txt
<http://uricorn.fly/tib_lok_sys#mat734_2>
filen.txt
<http://uricorn.fly/tib_lok_sys#mat1>
How can this be achieved?

Answer 1

Source data, with some extras added to the last 2 files:
$ for s in 1 2 n
do
fn="file${s}.txt"
echo "+++++++++++ ${fn}"
cat "${fn}"
done
+++++++++++ file1.txt
mat 445
+++++++++++ file2.txt
mat 734.2.3
+++++++++++ filen.txt
mat 1 2 3
FBW
One awk solution that implements the most recent set of question edits:
awk -i inplace ' # overwrite the source file
/^mat/ { gsub(/ /,"") # if line starts with "^mat" then remove spaces ...
gsub(/\./,"_") # and replace periods with underscores
printf "<http://uricorn.fly/tib_lok_sys#%s>\n", $0 # print the desired output
}
' file{1,2,n}.txt
NOTES:
the -i inplace option requires GNU awk 4.1.0 (or better)
remove comments to declutter code
The above generates the following:
$ for s in 1 2 n
do
fn="file${s}.txt"
echo "+++++++++++ ${fn}"
cat "${fn}"
done
+++++++++++ file1.txt
<http://uricorn.fly/tib_lok_sys#mat445>
+++++++++++ file2.txt
<http://uricorn.fly/tib_lok_sys#mat734_2_3>
+++++++++++ filen.txt
<http://uricorn.fly/tib_lok_sys#mat123>

Answer 2

Sed:
sed -ri '/^mat/{s/[ ]//g;s/[.]/_/g;s#^(.*)$#<http://uricorn.fly/tib_lok_sys#\1>#g}' *.txt
Search for lines starting with mat and then first remove spaces, replace . with _ and finally substitute this string with a string including the http string prepended.

Answer 3

The other answers are far more elegant, but none worked on my system so here is what did eventually:
for f in *.txt ; do
# Remove every line that does not contain 'mat'
sed -i '/mat/!d' $f
# Remove every character until 'mat' begins
sed -i -e 's/^.*\(mat.*\).*$/\1/' $f
# Remove the blank between 'mat' and number
sed -i -e 's/ //g' $f
# Replace the dot in subcategories with an underscore
tr '.' '_' < $f
# Add URI
sed -i -e 's/^/\<http:\/\/uricorn.fly\/tib\_lok\_sys\#/' "$f"
sed -i -e 's/\(.*\)[0-9]/&> /' "$f"
uniq $f
done