Removal of lines with no more or fewer than 'N' fields?

I am working on mac with sed, perl, awk, bash..

I have a large-ish (10GB) text file which has 13 fields (columns) of TAB delimited data. Unfortunately some of these lines have extraneous TABs , so I want to delete the entire line where we have extra TABs , and thus unequal fields. (I don’t mind discarding the lines in their entirety)

What I currently have writes the number of fields into another file.

awk -F't' '{print NF}' infile  > fieldCount

head fieldCount

I would like to construct a short script that removes any line with more (or less) than 13 proper fields (from the original file).

  1. speed is helpful as I have to do this on multiple files
  2. doing it in one sweep would be cool
  3. I currently am porting the fieldCount file into Python, trying to load with line by line.


vaild (13 columns)

a       b       c       d       e       f       g       h       i       j       k       l       m

invalid (14 columns)

a       b       c       d       e       f       g       h       i       j       k       l       m       n
Asked By: T. Scharf


You almost have it already:

awk -F't' 'NF==13 {print}' infile  > newfile

And, if you’re on one of those systems
where you’re charged by the keystroke ( 🙂 )
you can shorten that to

awk -F't' 'NF==13' infile  > newfile

To do multiple files in one sweep,
and to actually change the files (and not just create new files),
identify a filename thats not in use (for example, scharf),
and perform a loop, like this:

for f in list
    awk -F't' 'NF==13 {print}' "$f" > scharf  &&  mv -f -- scharf "$f"

The list can be one or more filenames
and/or wildcard filename expansion patterns; for example,

for f in *.dat /ultra/violet.dat

The mv command overwrites the input file (e.g.,
with the temporary scharf file
(which has only the lines from the input file with 13 fields). 
(Be sure this is what you want to do, and be careful. 
To be safe, you should probably back up your data first.) 
The -f tells mv to overwrite the input file,
even though it already exists. 
The -- protects you against weirdness
if any of your files has a name beginning with -.

With perl:

perl -F't' -anle 'print if @F == 13' file

to edit inplace, add -i option:

perl -i.bak -F't' -anle 'print if @F == 13' file
Answered By: cuonglm

Since this is a large file, it may be worth using a slightly more complex tool for a performance gain. Usually, specialized tools are faster than generalist tools. For example, solving the same problem with cut tends to be faster than grep which tends to be faster than sed which tends to be faster than awk (the flip side being that later tools can do things that earlier ones can’t).

You want to remove lines with 13 tab characters or more, so:

LC_ALL=C grep -Ev '(␉.*){13}'

or maybe (I don’t expect a measurable performance difference)

LC_ALL=C grep -Ev '(␉.*){12}␉'

where is a literal tab character. Setting the locale to C isn’t necessary, but speeds up some versions of GNU grep compared with multibyte locales.

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