How to wrap color coded text to fixed line length in terminal?

For text with color codes, how to wrap it to a fixed length in the terminal?

Text without color codes wraps nicely with fold:

echo -e "12345678901234567890" | fold -w 10
1234567890
1234567890

But this red text wraps wrong:

echo -e "u001b[31m12345678901234567890" | fold -w 10
12345
6789012345
67890

Note: While the red text is wrapped wrong, it still is printed in red, which is the desired behavior.

(My use case is line wrapping the output of git log --color=always --oneline --graph.)

Answer

When determining the (printable) width of a prompt (eg, PS1) the special characters – [ and ] – are used to designate a series of non-printing characters (see this, this, this and this).

So far I’ve been unable to find a way to use [ and ] outside the scope of a prompt hence this awk hack …

---

Assumptions:

• we don’t know the color codes in advance
• for this exercise it is sufficient to deal with color codes of the format e[...m (e[m turns off color)
• may have to deal with multiple color codes in the input

We’ll wrap one awk idea in a bash function (for easier use):

myfold() {

awk -v n="${1:-10}" '                                                  # default wrap is 10 (printable) characters
BEGIN { regex="[[:cntrl:]][[][^m]*m"                                   # regex == "e[*m"
       #regex="x1b[[][^m]*m"                                          # alternatives
       #regex="33[[][^m]*m"
      }
      { input=$0

        while (input != "" ) {                                         # repeatedly strip off "n" characters until we have processed the entire line
              count=n
              output=""

              while ( count > 0 ) {                                    # repeatedly strip off color control codes and characters until we have stripped of "n" characters
                    match(input,regex)

                    if (RSTART && RSTART <= count) {
                       output=output substr(input,1,RSTART+RLENGTH-1)
                       input=substr(input,RSTART+RLENGTH)
                       count=count - (RSTART > 1 ? RSTART-1 : 0)
                    }
                    else {
                       output=output substr(input,1,count)
                       input=substr(input,count+1)
                       count=0
                    }
              }
              print output
        }
      }
'
}

NOTES:

• other non-color, non-printing characters will throw off the count
• the regex could be expanded to address other non-printing color and/or character codes

Test run:

$ echo -e "e[31m123456789012345e[m67890e[32mABCDe[m"
12345678901234567890ABCD


$ echo -e "e[31m123456789012345e[m67890e[32mABCDe[m" | myfold 10
1234567890
1234567890
ABCD


$ echo -e "e[31m123456789012345e[m67890e[32mABCDe[m" | myfold 7
1234567
8901234
567890A
BCD

Displaying colors: