How to search for a matching string in a file bottom-to-top without using tac? [closed] - shell

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I need to grep through a file, starting at the bottom of the file until I get to the first date that appears "2021-04-04", and then return that date. I don't want to start from the top and work my way down to the first line as there's thousands of lines in each file.
Example file contents:
random text on first line
random text on second line
2021-01-01
random text on fourth line
2021-02-03
random text on sixth line
2021-03-03
2021-04-04
Random text on ninth line
tac isn't available on MacOS so I can't use it.

Answer 1

"thousands of lines" are nothing, they'll be processed in the blink of an eye. Once you get into 10s of millions of lines THEN you could start thinking about a performance improvement if it became necessary.
All you need is:
awk '/[0-9]{4}(-[0-9]{2}){2}/{line=$0} END{if (line!="") print line}' file
Here's the 3rd-run timing comparison for finding the last line containing 2 or more consecutive 5s in a 100000 line file generated by seq 100000 > file100k, i.e. where the target string is just 45 lines from the end of the input file, with and without tac:
$ time awk '/5{2}/{line=$0} END{if (line!="") print line}' file100k
99955
real 0m0.056s
user 0m0.031s
sys 0m0.000s
$ time tac file100k | awk '/5{2}/{print; exit}'
99955
real 0m0.056s
user 0m0.015s
sys 0m0.030s
As you can see, both ran in a fraction of a second and using tac did nothing to improve the speed of execution. Switching to tac+grep doesn't make it any faster either, it still just takes 1/20th of a second:
$ time tac file100k | grep -m1 '5\{2\}'
99955
real 0m0.057s
user 0m0.015s
sys 0m0.015s
In case you ever do need it in future, though, here's how to implement an efficient tac if you don't have it:
$ mytac() { cat -n "${#:--}" | sort -k1,1rn | cut -d$'\t' -f2-; }
$ seq 5 | mytac
5
4
3
2
1
The above mytac() function just adds line numbers to the input, sorts those in reverse and then removes them again. If your cat doesn't have -n to add line numbers then you can use nl if you have it or awk -v OFS='\t' '{print NR, $0}' will always work.

Answer 2

Use tac:
#!/bin/bash
function process_file_backwords(){
tac $1 | while IFS= read line; do
# Grep for xxxx-xx-xx number matching
first_date=$(echo $line | grep '[0-9]\{4\}-[0-9]\{2\}-[0-9]\{2\}' | awk -F '"' '{ print $2}')
# Check if the variable is not empty, if yes break the loop
[ ! -z $first_date ] && echo $first_date && break
done
}
echo $(process_file_backwords $1)
Note: Make sure you add empty line at the of the file so tac will not concatenate the last two lines.
Note: Remove the awk part if the file contains strings without ".

Answer 3

On MacOS
You can use tail -r which will do the same thing as tac but you may have to supply the number of lines you want tail to output from your file. Something like this should work:
tail -r -n $(wc -l myfile.txt | cut -d ' ' -f 1) myfile.txt | grep -m 1 -o -P '\d{4}-\d{2}-\d{2}'
-r tells tail to output its last line first
-n takes a numeric argument telling how many lines tail should output
wc -l outputs the line count and filename of a given file
cut -d ' ' splits the above on the space character and -f 1 takes the first "field" which will be our line count
$ cat myfile.txt
foo
this is a date 2021-04-03
bar
this is another date 2021-04-04 for example
$ tail -r -n $(wc -l myfile.txt | cut -d ' ' -f 1) myfile.txt | grep -m 1 -o -P '\d{4}-\d{2}-\d{2}'
2021-04-04
grep options:
The -m 1 option will quit after the first result.
The -o option
will return only the string matching the pattern (i.e. your date)
The -P option uses the perl regex engine which is really down to
preference but I personally prefer the regex syntax (seems to use
fewer backslashes \)
On Linux
You can use tac (cat in reverse) and pipe that into your grep. e.g.:
$ tac myfile.txt
this is another date 2021-04-04 for example
bar
this is a date 2021-04-03
foo
$ tac myfile.txt | grep -m 1 -o -P '\d{4}-\d{2}-\d{2}'
2021-04-04

Answer 4

You can use perl to reverse the lines and grep for the 1st match too.
perl -e 'print reverse<>' inputFile | grep -m1 '[0-9]\{4\}-[0-9]\{2\}-[0-9]\{2\}'