Replace 2nd line in flat file

Question

I am new bee for shell script. Please redirect me to correct post if this question is redundant.

I have file.txt which has text like below and want to change the 2nd row in file

Property1=0
Property2=0 50 14 1/1 * ? *
Property3=0

I want to replace 2nd line to change 50 14 to current time, file will look like

Property1=0
Property2=0 58 15 1/1 * ? *
Property3=0

and next time it will look like

Property1=0
Property2=0 03 16 1/1 * ? *
Property3=0

Please help me how to change 2nd line.

What is the logic that governs what you're replacing and what the replacement is? — Barmar, Aug 11 '16 at 23:57
This is cron pattern, which I need to replace in flat file from shell. SO when I execute shell, time should change based on current time. — Shikha A, Aug 12 '16 at 00:06

score 3 · Answer 1 · edited Aug 11 '16 at 23:59

3

You can use sed to change the second line

sed '2s/.*/Property2=0 58 15 1\/1 * ? */' test

It seems like you're trying to increment, but I don't see the pattern in the third example

edited Aug 11 '16 at 23:59

Barmar

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answered Aug 11 '16 at 23:59

willpnw

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I think they're times. first number is minutes, second number is hours – Barmar Aug 12 '16 at 00:00
Yeah, looks like some sort of cron job... but how does 58 15 go to 3 16 – willpnw Aug 12 '16 at 00:02
The first time is at `15:58`, the next one is at `16:03` – Barmar Aug 12 '16 at 00:02
yes, this is cron job and I am trying to fill minutes and hours. Based on shell when it runs – Shikha A Aug 12 '16 at 00:05

score 2 · Answer 2 · answered Aug 12 '16 at 00:03

2

This is probably easier done using awk than sed.

awk -v min=$(date +%M) -v hour=$(date +%H) 'NR == 2 { $2 = min; $3 = hour } 1' file > file.new

The two date commands set awk variables to the current time. NR == 2 matches line 2 in the file, then it replaces the 2nd and 3rd fields with those time variables. 1 at the end causes the current line to be printed.

answered Aug 12 '16 at 00:03

Barmar

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SO you want me to create a new file "file.new" everytime I want to change this time? If this is the case, then I should go for sed – Shikha A Aug 12 '16 at 00:07
Recent versions of GNU awk have an `inplace` option to replace the input file. See http://stackoverflow.com/questions/16529716/awk-save-modifications-inplace – Barmar Aug 12 '16 at 00:08
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@ShikhaA even if your awk doesn't have inplace editing, typing `awk 'script' file > tmp && mv tmp file` really isn't that much more effort than typing `sed -i tmp 'script' file` especially when the sed script will be less concise than the awk script. – Ed Morton Aug 12 '16 at 00:15

Replace 2nd line in flat file

2 Answers2