Linux operating system

Question

I am a beginner at Linux and I'm trying to do a project which takes every line from a file.txt and replaces the third word with the first of each line. Here is my Shell code but it doesn't seem to work. It keeps replacing the third word with $field1 and not what's in it.

#!/bin/bash

while IFS=: read -r field1;do    
    sed -e 's/[^:]*[^:]/$field1/3'
done < file.txt

you might want to change the title of your question to something more relevant, like "using variable with sed not working" and in which case you might find this answer: http://stackoverflow.com/questions/19151954/how-to-use-variables-in-a-command-in-sed — Matthias, Apr 05 '17 at 10:38
Your `sed` command consumes the rest of `file.txt` because you are not specifying an input file. Use either a `while read` loop or `sed` (preferably the latter in most cases) but not both, unless they need to operate on separate streams or files. — tripleee, Apr 05 '17 at 10:50

score 0 · Answer 1 · edited Apr 05 '17 at 10:44

0

Make note of the single quotation marks. Place them around the field1 variable and so:

sed -e 's/[^:]*[^:]/'$field1'/3'

edited Apr 05 '17 at 10:44

Nathan Fellman

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answered Apr 05 '17 at 10:36

Raman Sailopal

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Nope. I don't know why but it only duplicates the fields. Not even replacing anything. – TdFn Apr 05 '17 at 10:44

Ashish K · Accepted Answer · 2017-04-05T11:20:08.487

0

Try this, this will replace in the same file:

#!/bin/sh

while read -r line
do
        first=`echo $line | awk -F':' '{ print $1 }'`
        last=`echo $line | awk -F':' '{ print $3 }'`

        echo $line | sed "s/$last/$first/"

done < file.txt

Input file :

ashish:is:good
navin:is:good
how:are:you

Output :

ashish:is:ashish
navin:is:navin
how:are:how

edited Apr 05 '17 at 11:20

answered Apr 05 '17 at 10:43

Ashish K

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It doesn't help if I do that. I have to replace the third word with the first of each line. That bit will only print the fields. – TdFn Apr 05 '17 at 10:46
replace, not exchange? correct? – Ashish K Apr 05 '17 at 10:48
ok, that works. but ultimately i have to replace those words in the file. not just print them on screen. – TdFn Apr 05 '17 at 10:50
Your quoting is exactly wrong. The colons are not shell metacharacters and thus do not require quoting, while the variables could contain shell metacharacters, irregular whitespace, etc and thus *need* to be quoted. (As an experiment, use input where one of the fields is an asterisk and nothing else to see the point proven.) – tripleee Apr 05 '17 at 10:52
Quoting is not wrong but shouldn't have been used. Anyway thanks for mentioning. – Ashish K Apr 05 '17 at 10:56
@TdFn please check now. Tell me if you are still facing any issue. – Ashish K Apr 05 '17 at 11:20
@AshishK Great! It's perfect. Thank you very much! – TdFn Apr 05 '17 at 11:37

Linux operating system

2 Answers2