Spaces are used to separate the elements of the list that we are going to loop over. If one of those elements contains a space character, we need to surround it with quotes, and do the same thing to our loop variable. Suppose our data files are named:
red dragon.dat
purple unicorn.dat
To loop over these files, we would need to add double quotes like so:
$ for filename in "red dragon.dat" "purple unicorn.dat"
do
head -n 100 "$filename" | tail -n 20
done
It is simpler to avoid using spaces (or other special characters) in filenames.
The files above don’t exist, so if we run the above code, the head command will be unable
to find them, however the error message returned will show the name of the files it is
expecting:
head: cannot open ‘red dragon.dat’ for reading: No such file or directory
head: cannot open ‘purple unicorn.dat’ for reading: No such file or directory
Try removing the quotes around $filename in the loop above to see the effect of the quote
marks on spaces. Note that we get a result from the loop command for unicorn.dat
when we run this code in the creatures directory:
head: cannot open ‘red’ for reading: No such file or directory
head: cannot open ‘dragon.dat’ for reading: No such file or directory
head: cannot open ‘purple’ for reading: No such file or directory
CGGTACCGAA
AAGGGTCGCG
CAAGTGTTCC
...