Ygtjki

Is it possible to replace only "leading tabs" with a set number of spaces per tab? I'm defining a leading tab as one that is only preceded by tabs or spaces. I'm defining a leading or an indentation tab as one that is only preceded by whitespace.

I have the following line in my .nexrc, cribbed from this one. It runs the lines between the lines containing the mark a and the mark b through a filter. that replaces tabs.

map 2 'a!'b pr -te2

However, it replaces non-leading tabs with two spaces well, which is not ideal since a literal tab is sometimes needed.

Is there an easy and portable way to replace leading tabs only with a set number of spaces per tab?

The following perl script (which could be collapsed to a one-liner) does the job of replacing leading tabs with a width supplied on the command line, but it relies on the availability of Perl and takes quadratic time (if the file consists of nothing but tabs).

#!/usr/bin/env perl

use strict;
use warnings;

my ($width) = @ARGV;
$width //= 2;
my $replacement = ' ' x $width;

while (<>) 
 while (s/^(s*)t/$1$replacement/g) 
 print;

Here's an example of what I want to happen. ^I refers to a tab byte and # is a zero-width marker for the start of a line. The line below is a byte-ruler.

BEFORE:

#^I^I ^Ia^Ib
# 1 23 45 67

AFTER (tab width of 2)

# a^Ib
#12345678 90

AFTER (tab width of 4)

# a^Ib
#12345678901234 56

If you are looking for a generic solution using other command-line tools, one could use Awk for this.

var=$'tttnewcontent'
echo "$var" | awk ' gsub(/^[t]+/," ",$0) 1'

The command above replaces one more more occurrences of Tab with a single space i.e. 2^nd argument in gsub. You can make it configurable though by generating how many spaces you want

awk -v n=5 ' BEGIN spaces = sprintf("%"n"s"," ") gsub(/^[t]+/,spaces,$0) 1'

From the last update, it seems OP wants to replace the number of leading tabs with the exact number of spaces found,

awk -F$'t' ' spaces = sprintf("%"(NF-1)"s"," "); gsub(/^[t]+/,spaces,$0) 1'

The logic is with setting field separator to t, the number of leading tabs can be identified by doing NF-1. With the number of leading tabs identified, use it to generate the same of number of spaces from earlier sprintf() method.

You can see it by verifying the hexdump -C before and after the replacement

echo "$var" | hexdump -C
00000000 09 09 09 6e 65 77 63 6f 6e 74 65 6e 74 0a |...newcontent.|
0000000e

now with the replacement

echo "$var" | awk -F$'t' ' spaces = sprintf("%"(NF-1)"s"," "); gsub(/^[t]+/,spaces,$0) 1' | hexdump -C
00000000 20 20 20 6e 65 77 63 6f 6e 74 65 6e 74 0a | newcontent.|
0000000e