sed - replace string with file contents

Here is a sed solution:

% sed -e "s/localhost/$(sed 's:/:\\/:g' file2)/" file1
---
  host: "1.1.1.1"
  port: 3000
  reporter_type: "zookeeper"
  zk_hosts: 
    - "1.1.1.1:2181"

You should use sed -i to make the change inplace.

If you can use awk, here is one way to do:

% awk 'BEGIN{getline l < "file2"}/localhost/{gsub("localhost",l)}1' file1
---
  host: "1.1.1.1"
  port: 3000
  reporter_type: "zookeeper"
  zk_hosts: 
    - "1.1.1.1:2181"

You can read the file with the replacement string using shell command substitution, before sed is used. So sed will see just a normal substitution:

sed "s/localhost/$(cat file2)/" file1 > changed.txt

I also had this "problem" today: how to replace a block of text with the contents from another file.

I've solved it by making a bash function (that can be reused in scripts).

[cent@pcmk-1 tmp]$ cat the_function.sh
# This function reads text from stdin, and substitutes a *BLOCK* with the contents from a FILE, and outputs to stdout
# The BLOCK is indicated with BLOCK_StartRegexp and BLOCK_EndRegexp
#
# Usage:
#    seq 100 110 | substitute_BLOCK_with_FILEcontents '^102' '^104' /tmp/FileWithContents > /tmp/result.txt
function substitute_BLOCK_with_FILEcontents {
  local BLOCK_StartRegexp="${1}"
  local BLOCK_EndRegexp="${2}"
  local FILE="${3}"
  sed -e "/${BLOCK_EndRegexp}/a ___tmpMark___" -e "/${BLOCK_StartRegexp}/,/${BLOCK_EndRegexp}/d" | sed -e "/___tmpMark___/r ${FILE}" -e '/___tmpMark___/d'
}

[cent@pcmk-1 tmp]$
[cent@pcmk-1 tmp]$
[cent@pcmk-1 tmp]$ cat /tmp/FileWithContents
We have deleted everyhing between lines 102 and 104 and
replaced with this text, which was read from a file
[cent@pcmk-1 tmp]$
[cent@pcmk-1 tmp]$
[cent@pcmk-1 tmp]$ source the_function.sh
[cent@pcmk-1 tmp]$ seq 100 110 | substitute_BLOCK_with_FILEcontents '^102' '^104' /tmp/FileWithContents > /tmp/result.txt
[cent@pcmk-1 tmp]$
[cent@pcmk-1 tmp]$
[cent@pcmk-1 tmp]$ cat /tmp/result.txt
100
101
We have deleted everyhing between lines 102 and 104 and
replaced with this text, which was read from a file
105
106
107
108
109
110

Try using

join file1 file2

and then, remove any unwanted fields.