替换指定字符数

Question

I have something like this:

aaaaaaaaaaaaaaaaaaaaaaaaa

I need something that will allow me to replace a with another character like c from left to right according to the specified number.

For example:

some_command 3 should replace the first 3 a with c
cccaaaaaaaaaaaaaaaaaaaaaa

some_command 15
cccccccccccccccccaaaaaaaaaa

Answer 1

This can be done entirely in bash:

some_command() {
    a="aaaaaaaaaaaaaaaaaaaaaaaaa"
    c="ccccccccccccccccccccccccc"
    echo "${c:0:$1}${a:$1}"
}

> some_command 3
cccaaaaaaaaaaaaaaaaaaaaaa

Answer 2

Using awk:

s='aaaaaaaaaaaaaaaaaaaaaaaaa'
awk -F "\0" -v n=3 -v r='c' '{for (i=1; i<=n; i++) $i=r}1' OFS= <<< "$s"
cccaaaaaaaaaaaaaaaaaaaaaa

Answer 3

This might work for you (GNU sed):

sed -r ':a;/a/{x;/^X{5}$/{x;b};s/$/X/;x;s/a/c/;ba} file

This will replace the first 5 a 's with c throughout the file :

sed -r ':a;/a/{x;/^X{5}$/{z;x;b};s/$/X/;x;s/a/c/;ba} file

This will replace the first 5 a 's with c for each line throughout the file .

Answer 4

#/bin/bash

char=c
word=aaaaaaaaaaaaaaaaaaaaaaaaa

# pass in the number of chars to replace
replaceChar () {
   num=$1
   newword=""
   # this for loop to concatenate the chars could probably be optimized
   for i in $(seq 1 $num); do newword="${newword}${char}"; done
   word="${newword}${word:$num}"
   echo $word
}

replaceChar 4

Answer 5

A more general solution than the OP asked for, building on @anubhava's excellent answer.

• Parameterizes the replacement count as well as the "before and after" chars.
• The "before" char is matched anywhere - not just at the beginning of the input string, and whether adjacent to other instances or not.
• Input is taken from stdin , so multiple lines can be piped in.

# Usage:
# ... | some_command_x replaceCount beforeChar afterChar
some_command_x() { 
  awk -F '\0' -v n="$1" -v o="${2:0:1}" -v r="${3:0:1}" -v OFS='' \
    '{
      while(++i <= NF)
       { if ($i==o) { if (++n_matched > n) break; $i=r } }
     { i=n_matched=0; print }
    }'
}

# Example:
some_command_x 2 a c <<<$'abc_abc_abc\naaa rating'

# Returns:
cbc_cbc_abc
cca rating

Answer 6

Perl has some interesting features that can be exploited. Define the following bash script some_command :

#! /bin/bash
str="aaaaaaaaaaaaaaaaaaaaaaaaa"
perl -s -nE'print s/(a{$x})/"c" x length $1/er' -- -x=$1 <<<"$str"

Testing:

$ some_command 5
cccccaaaaaaaaaaaaaaaaaaaa