I'm trying to extract a certain (the fourth) field from the column-based, 'space'-adjusted text stream. I'm trying to use the cut command in the following manner:
cat text.txt | cut -d " " -f 4
Unfortunately, cut doesn't treat several spaces as one delimiter. I could have piped through awk
awk '{ printf $4; }'
or sed
sed -E "s/[[:space:]]+/ /g"
to collapse the spaces, but I'd like to know if there any way to deal with cut and several delimiters natively?
Try:
tr -s ' ' <text.txt | cut -d ' ' -f4
From the tr man page:
-s, --squeeze-repeats replace each input sequence of a repeated character
that is listed in SET1 with a single occurrence
of that character
As you comment in your question, awk is really the way to go. To use cut is possible together with tr -s to squeeze spaces, as kev's answer shows.
Let me however go through all the possible combinations for future readers. Explanations are at the Test section.
tr -s ' ' < file | cut -d' ' -f4
awk '{print $4}' file
while read -r _ _ _ myfield _
do
echo "forth field: $myfield"
done < file
sed -r 's/^([^ ]*[ ]*){3}([^ ]*).*/\2/' file
Given this file, let's test the commands:
$ cat a
this is line 1 more text
this is line 2 more text
this is line 3 more text
this is line 4 more text
$ cut -d' ' -f4 a
is
# it does not show what we want!
$ tr -s ' ' < a | cut -d' ' -f4
1
2 # this makes it!
3
4
$
$ awk '{print $4}' a
1
2
3
4
This reads the fields sequentially. By using _ we indicate that this is a throwaway variable as a "junk variable" to ignore these fields. This way, we store $myfield as the 4th field in the file, no matter the spaces in between them.
$ while read -r _ _ _ a _; do echo "4th field: $a"; done < a
4th field: 1
4th field: 2
4th field: 3
4th field: 4
This catches three groups of spaces and no spaces with ([^ ]*[ ]*){3}. Then, it catches whatever coming until a space as the 4th field, that it is finally printed with \1.
$ sed -r 's/^([^ ]*[ ]*){3}([^ ]*).*/\2/' a
1
2
3
4
After becoming frustrated with the too many limitations of cut, I wrote my own replacement, which I called cuts for "cut on steroids".
cuts provides what is likely the most minimalist solution to this and many other related cut/paste problems.
One example, out of many, addressing this particular question:
$ cat text.txt
0 1 2 3
0 1 2 3 4
$ cuts 2 text.txt
2
2
cuts supports:
paste separately)and much more. None of which is provided by standard cut.
See also: https://stackoverflow.com/a/24543231/1296044
Source and documentation (free software): http://arielf.github.io/cuts/
This Perl one-liner shows how closely Perl is related to awk:
perl -lane 'print $F[3]' text.txt
However, the @F autosplit array starts at index $F[0] while awk fields start with $1
With versions of cut I know of, no, this is not possible. cut is primarily useful for parsing files where the separator is not whitespace (for example /etc/passwd) and that have a fixed number of fields. Two separators in a row mean an empty field, and that goes for whitespace too.
I've created a patch that adds new -m command-line option to cut, which works in the field mode and treats multiple consecutive delimiters as a single delimiter. This basically solves the OP's question in a rather efficient way. I also submitted this patch upstream a couple of days ago, and let's hope that it will be merged into the coreutils project.
There are some further thoughts about adding even more whitespace-related features to cut, and having some feedback about all that would be great. I'm willing to implement more patches for cut and submit them upstream, which would make this utility more versatile and more usable in various real-world scenarios.
