MT#64932 Do not use which(1) output unconditionally

Instead of using a variable to hold the pathname for the mysql program,
from a which(1) call, hardcode its name in the variable, and then use
«command -v» to check whether the command exists and abort otherwise.

The pattern of assigning the result of which(1) into a variable is both
redundant and unsafe, because the shell already looks for the command on
the PATH, and if which(1) cannot find it then we are then executing the
command unconditionally where we end up calling the first argument as if
it was an actual command, which can be dangerous.

Change-Id: I6c31e03b8b20fe037869f2f41954f5ed268434ca

ngcp-cleanup-voicemail-table

@@ -10,7 +10,7 @@ set -eu
 PID_FILE='/var/lock/voicemail-cleanup.pid'
 PWD_FILE='/etc/mysql/sipwise_extra.cnf'
 CFG_FILE='/etc/ngcp-cleanup-tools/voicemail-table-cleanup.conf'
-MYSQL="$(which mysql)"
+MYSQL=mysql
 QUERY_COUNT=1
 
 #Duplicating output to logs
@@ -27,6 +27,11 @@ my_exit() {
 
 echo " $(date): Starting the new process"
 
+if ! command -v "${MYSQL}" >/dev/null ; then
+  echo "ERROR: Cannot find ${MYSQL}!" >&2
+  exit 1
+fi
+
 if [ -e "${PID_FILE}" ] ; then
   echo "ERROR: The lock file exists." >&2
   echo "The previous instance has not exited correctly or still running? Exiting!" >&2