Cutting characters from certain lines if condition is met, otherwise printing whole line

I want to cut the characters n-N of a line, but ONLY if the line begins with certain characters, otherwise I want to print the whole line.

Simplified File example:

John
123456987123
Mark
123546792019

I want to make two new files, one with the FIRST 6 numbers and the other with the last 6 numbers, but still containing the headers, so

John
123456
Mark
123546

---

John
987123
Mark
792019

Can I tell grep cut to only cut if the string matches, but to otherwise give the whole file? What sort of awk command can cut lines if a condition is met or otherwise print the whole line?

Tha开发者_Go百科nks

---

Grep can only grep, not cut.

awk '/^[0-9]/{print(substr($0,0,6));next;}
        # Fall through here in case of no match
        {print}' inputfile

---

You can generate your 2 files in a single sed script, reading your input file only once. Put what is bellow in a text file, and run is with sed -f script_file input_file. It will generate the f1 and f2 files and output the input file.

/^[^0-9]/{
w f1
w f2
}
/^[0-9]/{
h
s/^\(.\{6\}\).*/\1/
w f1
g
s/^.\{6\}//
w f2
g}

---

grep should be fine:

grep -E -o '^[0-9]{6}|^[^0-9].*' input > first.txt
grep -E -o '[0-9]{6}$|^[^0-9].*' input > last.txt