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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
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  • 2
    Are you looking for an nvi solution, or a general command-line solution? Commented Mar 28, 2019 at 7:16
  • 1
    expand (from GNU coreutils) would seem to be the obvious generic solution e.g. expand -it3 to replace each initial tab with 3 spaces Commented Mar 28, 2019 at 11:03
  • sed 's/^\t\t*/ /' replace leading tabs with 2 spaces Commented Mar 28, 2019 at 15:53
  • sorry, I meant replacing leading tabs with two spaces per tab. Commented Mar 28, 2019 at 15:59
  • @GregoryNisbet If you want i read it add @ before my name. You can try sed ':A;s/(^\t)(\t*)/\2 /;tA' Commented Mar 28, 2019 at 16:17

1 Answer 1

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

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

The command above replaces one more more occurrences of Tab with a single space i.e. 2nd 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
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