Modify config file using bash script

Question

A wild stab in the dark for modifying a single value:

sed -c -i "s/\($TARGET_KEY *= *\).*/\1$REPLACEMENT_VALUE/" $CONFIG_FILE

assuming that the target key and replacement value don't contain any special regex characters, and that your key-value separator is "=". Note, the -c option is system dependent and you may need to omit it for sed to execute.

For other tips on how to do similar replacements (e.g., when the REPLACEMENT_VALUE has '/' characters in it), there are some great examples here.

Hope this helps someone. I created a self contained script, which required config processing of sorts.

#!/bin/bash
CONFIG="/tmp/test.cfg"

# Use this to set the new config value, needs 2 parameters. 
# You could check that $1 and $2 is set, but I am lazy
function set_config(){
    sudo sed -i "s/^\($1\s*=\s*\).*\$/\1$2/" $CONFIG
}

# INITIALIZE CONFIG IF IT'S MISSING
if [ ! -e "${CONFIG}" ] ; then
    # Set default variable value
    sudo touch $CONFIG
    echo "myname=\"Test\"" | sudo tee --append $CONFIG
fi

# LOAD THE CONFIG FILE
source $CONFIG

echo "${myname}" # SHOULD OUTPUT DEFAULT (test) ON FIRST RUN
myname="Erl"
echo "${myname}" # SHOULD OUTPUT Erl
set_config myname $myname # SETS THE NEW VALUE

Assuming that you have a file of key=value pairs, potentially with spaces around the =, you can delete, modify in-place or append key-value pairs at will using awk even if the keys or values contain special regex sequences:

# Using awk to delete, modify or append keys
# In case of an error the original configuration file is left intact
# Also leaves a timestamped backup copy (omit the cp -p if none is required)
CONFIG_FILE=file.conf
cp -p "$CONFIG_FILE" "$CONFIG_FILE.orig.`date \"+%Y%m%d_%H%M%S\"`" &&
awk -F '[ \t]*=[ \t]*' '$1=="keytodelete" { next } $1=="keytomodify" { print "keytomodify=newvalue" ; next } { print } END { print "keytoappend=value" }' "$CONFIG_FILE" >"$CONFIG_FILE~" &&
mv "$CONFIG_FILE~" "$CONFIG_FILE" ||
echo "an error has occurred (permissions? disk space?)"

sed "/^$old/s/\(.[^=]*\)\([ \t]*=[ \t]*\)\(.[^=]*\)/\1\2$replace/" configfile

So I can not take any credit for this as it is a combination of stackoverflow answers and help from irc.freenode.net #bash channel but here are bash functions now to both set and read config file values:

# https://stackoverflow.com/a/2464883
# Usage: config_set filename key value
function config_set() {
  local file=$1
  local key=$2
  local val=${@:3}

  ensureConfigFileExists "${file}"

  # create key if not exists
  if ! grep -q "^${key}=" ${file}; then
    # insert a newline just in case the file does not end with one
    printf "\n${key}=" >> ${file}
  fi

  chc "$file" "$key" "$val"
}

function ensureConfigFileExists() {
  if [ ! -e "$1" ] ; then
    if [ -e "$1.example" ]; then
      cp "$1.example" "$1";
    else
      touch "$1"
    fi
  fi
}

# thanks to ixz in #bash on irc.freenode.net
function chc() { gawk -v OFS== -v FS== -e 'BEGIN { ARGC = 1 } $1 == ARGV[2] { print ARGV[4] ? ARGV[4] : $1, ARGV[3]; next } 1' "$@" <"$1" >"$1.1"; mv "$1"{.1,}; }

# https://unix.stackexchange.com/a/331965/312709
# Usage: local myvar="$(config_get myvar)"
function config_get() {
    val="$(config_read_file ${CONFIG_FILE} "${1}")";
    if [ "${val}" = "__UNDEFINED__" ]; then
        val="$(config_read_file ${CONFIG_FILE}.example "${1}")";
    fi
    printf -- "%s" "${val}";
}
function config_read_file() {
    (grep -E "^${2}=" -m 1 "${1}" 2>/dev/null || echo "VAR=__UNDEFINED__") | head -n 1 | cut -d '=' -f 2-;
}

at first I was using the accepted answer's sed solution: https://stackoverflow.com/a/2464883/2683059

however if the value has a / char it breaks