How to remove a part of a column using awk

• awk

  awk -F- '$0=$1' file
  

• cut

  cut -d- -f1 file
  

• sed

  sed 's/-.*//' file
  

• perl

  perl -pe 's/-.*//' file
  

Simply with awk:

awk -F'-' '{ print $1 }' file

• -F'-' - treat - (dash) as field separator

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But in your simple case grep approach would be even simpler:

grep -o '^[^-]*' file

If the input only contains the timestamps, then it's easy to set the dash as the field separator and only print the first field:

$ awk -F- '{print $1}' input
2018:01:02
2018:01:02

But if you also have something else in there, say input2 contains

2018:01:02-23:52:48 some data 
2018:01:02-23:52:48 something else

then that would drop the rest of the line, and for other processing, you might not want to change the field separator either. But you can do a simple substitution on the first field and print the resulting line:

$ awk '{sub(/-.*/, "", $1)} 1' input2
2018:01:02 some data
2018:01:02 something else