Replace column values in a CSV file with awk

Question

I have this file

error.log

[00:00:00.284],501,

[00:00:00.417],5,5294100071980

[00:00:02.463],501,

[00:00:05.169],501,

[00:00:05.529],501,

[00:00:05.730],501,

so, if the field $3 its empty i want to print "No value"

Im trying this code

awk '{{FS=","} if($3=="") {print $1,$2,"No value"}}'

But it prints

>[00:00:00.284] 501 No value
>[00:00:02.463] 501 No value
>[00:00:05.169] 501 No value
>[00:00:05.529] 501 No value
>[00:00:05.730] 501 No value
>[00:00:07.193] 501 No value
>[00:00:09.899] 501 No value
>[00:00:31.312] 501 No value

Answer 1

awk -F ',' -v OFS=',' '$1 { if ($3=="") $3="No value"; print}' in.txt

• Passes the field separator via the -F option.
• Variable OFS, the output-field separator, is set to ,, so that output fields are also separated by ,.
• Pattern $1 ensures that only non-empty lines are processed (that is, the associated action is only executed if the first field is non-empty) - if your input file has no empty lines, you can remove this pattern.
• If the 3rd field is empty, it is assigned string "No value"
• Finally, the line (with the potentially modified 3rd field) is output.

The above is how I suggest you approach the problem, but here are the problems with your original command:

• {{FS=","}... Inside your single action - which due to not having a preceding pattern is executed for every input line - you set variable FS for every line - which is not only unnecessary but too late, because the first input line has already been parsed by that time (thanks, @EdMorton) - either set it in a BEGIN block (BEGIN { FS="," }) or, as in my answer, with command-line option -F (-F ',').
• if($3=="") {...}
  You only produce output if field $3 is empty - presumably, though, you want to output all lines, so with this approach you'd need an else branch (to print unmodified lines).
• print $1,$2,"No value"
  The , chars. here are part of the syntax - they simply separate the arguments passed to print. Given separate arguments, print concatenates them with the value of the special OFS variable, whose value is a single space by default; to use , instead, you have to assign it to OFS - again, either in a BEGIN block or via the -v option (-v OFS=',').

Answer 2

You should post some expected output but I THINK what you want is:

awk 'BEGIN{FS=OFS=","} NF{print $1, $2, ($3=="" ? "No value" : $3)}' file

Answer 3

With this file

cat file
[00:00:00.284],501,
[00:00:00.417],5,5294100071980
[00:00:02.463],501,
[00:00:05.169],501,
[00:00:05.529],501,
[00:00:05.730],501,

This awk should do

awk -F, '$3=="" {$3="No value"}1' OFS=, file
[00:00:00.284],501,No value
[00:00:00.417],5,5294100071980
[00:00:02.463],501,No value
[00:00:05.169],501,No value
[00:00:05.529],501,No value
[00:00:05.730],501,No value