Given an input like this
this,is,a,test,string,containing,multiple
lines,of,string,with,numb3rs,and,w0rds
I want to delete every second and fourth word in each line using sed. Words are strictly alphanumeric.
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"Every 2nd word" would include the 4th word. Do you mean "the 2nd and 4th word on each line"?– Kusalananda ♦Apr 19, 2020 at 9:12
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Sorry, I did not specify more precisely. I meant exactly the 2nd and 4th word on each line.– Paralyz3dApr 19, 2020 at 9:15
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Do you want to remove only the 2nd/4th word even though a field may contain multiple words? Or does a field only ever contain a single word?– Kusalananda ♦Apr 19, 2020 at 9:42
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1Should the separating commas be removed, too? Please add a sample output.– FelixJNApr 19, 2020 at 11:33
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Please show us your expected output. "Every second word" means you want to delete the 2nd, 4th, 6th, 8th etc. "Every fourth" would mean deleting the 4th, 8th, 12th, 16th etc. Without an example output it's very hard to understand what you mean.– terdon ♦Apr 19, 2020 at 13:12
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5 Answers
The most natural tool for this is cut.
cut -d , -f 1,3,5-
With sed, use \([^,]*,\) to match one field.
sed 's/^\([^,]*,\)\([^,]*,\)\([^,]*,\)\([^,]*,\)/\1\3/'
answered Apr 19, 2020 at 9:25
Gilles 'SO- stop being evil'Gilles 'SO- stop being evil'
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Or
sed 's/^\([^,]*,\)[^,]*,\([^,]*,\)[^,]*,/\1\2/'. Note that thesedones assume all lines have at least 5 fields.sed 's/,[^,]*//3; s/,[^,]*//'would take care of those. Apr 19, 2020 at 12:19 -
What if you want the second and the 113th ? Should you write a very long request?– SandburgOct 6, 2020 at 13:28
it's not sed, but you can use Miller (https://github.com/johnkerl/miller) and run
<input mlr --csv -N unsparsify then cut -x -f 2,4
to have
this,a,string,containing,multiple
lines,string,numb3rs,and,w0rds
If you just want to remove the 2nd and 4th fields on each line, you can do:
$ perl -F, -lane 'print join ",", @F[0,2,4..$#F]' file
this,a,string,containing,multiple
lines,string,numb3rs,and,w0rds
The -n tells perl to read an input file and apply the script given by -e to each line. The -a causes perl to act like awk and split its input on the character given by -F and save the result in the array @F. Then, join ",",@F[0,2,4..$#F]' makes a new string by joining the 1st and 3rd fields (arrays start from 0) and then the 5th field and everything else until the end of the array ($#F is the highest index in the array), and the print print this string.
awk '{ split($0,a,","); delete a[4]; delete a[2]; for (i=1;i<=length(a); i++){ if(a[i]!="") printf "%s,", a[i] }; printf "\n";}' inputfile
This second one does not work, despite this line in man gawk:
Assigning a value to an existing field causes the whole record to be rebuilt when $0 is referenced. Similarly, assigning a value to $0 causes the record to be resplit, creating new values for the fields.
gawk 'BEGIN{ FS=","; OFS="," }{ $2=""; $4=""; a=$0; $0=a; print $0 }' inputfile
EDIT: Above does not work because of the FS and gow awk handles them, and that's why this works:
gawk 'BEGIN{ FS=","; OFS="," }{ gsub(FS $2,""); gsub(FS $4,""); print $0 }'
output:
this,a,test,containing,multiple
lines,string,with,and,w0rds
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@Cyrus, but this, indeed shorter, version will output
this,,a,,string,, so two commas after this, which is same as my last version.– LuukApr 19, 2020 at 12:18 -
awk 'BEGIN{FS=",";OFS=","}{$2=$4="\b";print $0}' file
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This works for this case, where inner fields are deleted, including the delimiters. But in general, the backspace character is moving the cursor back, when the output is rendered, not actually deleting content. Here, when
$2is rendered, the cursor goes one position back, and the next comma overwrites the previous one. But if you had to remove the last column and use$NF="\b", the last comma would remain, because there are no more characters to overwrite it. Or for other cases, it could have unexpected behaviour. Apr 18, 2022 at 10:04