I am trying to remove all occurrences of 2016/01/30 14:52:51: but the last one. I tried this:

awk '{gsub(//,"2016/01/30 14:52:51: ",$1);print}'

to replace all but the last occurrence with nothing, but that just duplicated 2016/01/30 14:52:51: 4 times with slightly different numbers.

  • I'm not sed-savvy enough, but I imagine if you could anchor the last occurrence of the string, you could do a search/replace/branch loop for the initial matches. e.g. search for "(.*)text(.*)text(ending pattern)$", replace with 1 & 3, repeat
    – Jeff Schaller
    Jan 30 '16 at 14:49
  • Are you trying to remove the occurrences in every line or the file. Could you give an example of your input, script and output.
    – jai_s
    Jan 30 '16 at 14:52

There is more than one problem with the script, plus the problem statement needs some clarification:

  • the gsub call has the regular expression in the wrong parameter
  • updating $1 has no effect on $0 (the value used in the print statement)
  • OP did not clarify if the intent was to leave the last occurrence on a line untouched, or only the last line containing the date (the latter is more likely).

Here is a script which incorporates those fixes and assumptions:

awk '
BEGIN { row=0; fixup = -1; }
    before[row] = $0;
    gsub("2016/01/30 14:52:51: ", "", $0);
    if ( $0 != before[row] ) {
            fixup = row;
    after[row++] = $0;
    if (fixup >= 0) {
            after[fixup] = before[fixup];
    for (n = 0; n < row; ++n) {
            print after[n];

(using two arrays is less efficient, but allows further modification with less effort than without the before array).

I tested this by making an input file (foo.in):

1awk '{gsub(//,"2016/01/30 14:52:51: ",$1);print}'
2awk '{gsub(//,"2016/01/30 14:52:51: ",$1);print}'
3awk '{gsub(//,"2016/01/30 14:52:51: ",$1);print}'
4awk '{gsub(//,"2016/01/30 14:52:51: ",$1);print}'

and running the script like this:

./foo <foo.in

and got

1awk '{gsub(//,"",$1);print}'
2awk '{gsub(//,"",$1);print}'
3awk '{gsub(//,"",$1);print}'
4awk '{gsub(//,"2016/01/30 14:52:51: ",$1);print}'
  • I actually didn't understand the script but it was someone who did the same as me and I simply replaced the values with my own. (Also, your answer doesn't seem to work for me, I tried this on a file with only two occurences and it's been running for 20 minutes with nothing happening. (I have restarted and ran again too, so it's not a one time problem). Jan 30 '16 at 15:43
  • You may have not redirected input to awk, e.g., the < character. Also, clarifying your problem with sample input would be helpful. Jan 30 '16 at 16:01

My contribution is a POSIX-compliant ed alternative to Thomas Dickey's AWK; it makes the same assumptions:

printf '%s\n' '1;?2016/01/30 14:52:51: ?' '1,.-g//s///' w q | ed filename

ed's ability to traverse the file backwards makes short work of this task.

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