[SOLVED] Replace string with another string based on backreference with sed

Issue

I'm trying to convert a predefined string %c# where # can be some number with another string. The catch is that the length of the other string must be truncated to # number of characters.

Ideally these set of commands would work:

FORMAT="%c10"
LAST_COMMIT="5189e42b14797b1e36ffb7fc5657c7eea08f1c0f"
echo $FORMAT | sed "s/%c\([0-9]\+\)/${LAST_COMMIT:0:\1}/g"

but clearly there is a syntax error on the \1. You can replace it with a number to see what I'm trying to get as output.

I'm open to using some other program other than sed to achieve this but ideally it should be programs that are pretty much native to most linux installations.

Thanks!

Solution

This is my idea.

echo ${LAST_COMMIT} | head -c $(echo ${FORMAT} | sed -e 's/%c//')

Get number with sed and get first some character with head.

EDIT1

This might be better.

echo ${LAST_COMMIT} | head -c $(echo ${FORMAT} | sed -e 's/%c\([0-9]\+\)/\1/')

EDIT2

I make the script because it is too tough to understand. Please try this.

$ cat sample.sh

#!/bin/bash

FORMAT="%b-%t-%c10-%c5"
LAST_COMMIT="5189e42b14797b1e36ffb7fc5657c7eea08f1c0f"

## List numbers
lengths=$(echo ${FORMAT} | sed -e "s/%[^c]//g" -e "s/-//g" -e "s/%c/ /g")

## Substitute %cXX to first XX characters of LAST_COMMIT
for n in ${lengths}
do
  to_str=$(echo ${LAST_COMMIT:0:${n}})
  FORMAT=$(echo ${FORMAT} | sed "s/%c${length}/${to_str}/")
done

## Print result
echo ${FORMAT}

This is the result.

$ ./sample.sh
%b-%t-5189e42b1410-5189e5

Also this is one line commands (Same contents but too long and too tough)

for n in $(echo ${FORMAT} | sed -e "s/%[^c]//g" -e "s/-//g" -e "s/%c/ /g"); do to_str=$(echo ${LAST_COMMIT:0:${n}}); FORMAT=$(echo ${FORMAT} | sed "s/%c${length}/${to_str}/"); done; echo ${FORMAT}

Answered By - AkihikoTakahashi