How to truncate the second column to given length

Question

Given input of the form

XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar foolkasjfdrte

how can I truncate only the second column? The delimiter is TAB and the second column must be at most 75 characters long.

Answer 1

Using awk, split the file using tabs and output the first field in full and the first 75 characters (at most) of the second:

awk -F "\t" 'BEGIN { OFS=FS }; { print $1, substr($2, 1, 75); }'

As pointed out by fedorqui, you can handle files with more than two fields by replacing the fields you need to truncate:

awk -F "\t" 'BEGIN { OFS=FS }; { $2=substr($2, 1, 75); print }'

You could apply the substr to multiple fields by looping over them if necessary.

Answer 2

If you want to print only the first 75 characters of the second column (including spaces, and assuming only two columns in the file), you can do:

$ perl -pe 's/(\t.{75}).*/$1/' file
XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar fool

Or, with GNU sed:

$ sed 's/\(.*\t.\{75\}\).*/\1/' file
XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar fool

Or:

$ sed -r 's/(.*\t.{75}).*/\1/' file
XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar fool

Alternatively, you could use fold, telling it to cut at the first 91 characters (that's 8 for the identifier and another 8 for the tab), and printing only the first line:

$ fold -w 91 file | head -n1
XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar fool

---

If your file can have more than 2 columns and you only want to truncate the second, you can do (which, as I just noticed, is just a rewording of Stephen's answer):

$ awk -F"\t" -vOFS="\t" '{$2=substr($2,1,75)}1;' file
XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar fool

Or (note that this will break if the first 75 characters of the 2nd column can be interpreted as a regular expression):

$ perl -F"\t" -pale 's/$F[1]/substr($F[1],0,75)/e' file
XY981743    foobarlkasdf saflkas asfZR!sgfad asdSAD asdsadf SAdfasdf46lk lksad bar fool

Answer 3

Portably/POSIXly with sed:

tab=$(printf '\t')
sed "s/\($tab[^$tab]\{0,75\}\)[^$tab]*/\1/"

Or to truncate every column:

sed "s/\([^$tab]\{75\}\)[^$tab]*/\1/g"

Answer 4

If there are only 2 columns:

sed -r 's/^([^\t]*\t)(.{0,75}).*/\1\2/'

{0,75} means select from 0 up to 75 chars.
.* is the removed section beyond char 75.

---

If there are 2 or more columns:

sed -r 's/^([^\t]*\t)([^\t]{0,75})[^\t]*(.*)/\1\2\3/' file

[^\t]* is the removed section beyond char 75.