Bash function that waits for string to appear in file.

waitforstring.sh

#!/bin/bash

# Waits for string $2 to appear in the file $1.
# Meanwhile it is printing output of the file $1.
# If err or ERR string is find in the file $1 then exits with error code 1.
# Example use:
#    waitforstring /var/log/myapp.log "MongoDB started"
function waitforstring {
    local LAST_LINE=1
    local READ_LOG
    local READ_LINES

    until fgrep -q "$2" $1; do 
        READ_LOG=`tail -n +${LAST_LINE} $1`
        if grep -q ERR $1; then
            tail -n +${LAST_LINE} $1
            exit 1
        fi
        if grep -q err $1; then
            tail -n +${LAST_LINE} $1
            exit 1
        fi

        if [[ ${READ_LOG} ]]; then
            READ_LINES=`echo "${READ_LOG}" | wc -l`
            LAST_LINE=$((LAST_LINE + READ_LINES))
            echo "${READ_LOG}"
        else 
            echo "Log is still the same. Retry in 5s..."
        fi
        sleep 5
    done

    tail -n +${LAST_LINE} $1
}