Linux scripts

=========================================================================
= Real-time log inspection and event handling. (Jason Chen, Yet Verified)
=========================================================================

#!/bin/sh

file=/tmp/qq

keyword="jason"

previous_line=0

while true; do

        if [ -f ${file} ]; then

                line=`wc -l ${file} | cut -f1 -d' '`

                if [ ${line} -ne ${previous_line} ]; then

                        matched=`tail -n 1 ${file} | grep ${keyword}`

                        if [ "${matched}" != "" ]; then

                                echo "action here"

                                previous_line=${line}

                        fi

                fi

        fi

        sleep 1

done 

=========================================================================
= Determine whether a process is running or not and make use it to make a conditional shell script?=========================================================================

REF https://askubuntu.com/questions/157779/how-to-determine-whether-a-process-is-running-or-not-and-make-use-it-to-make-a-c

#!/bin/bash

# Check if gedit is running
# -x flag only match processes whose name (or command line if -f is
# specified) exactly match the pattern. 

if pgrep -x "gedit" > /dev/null
then
    echo "Running"
else
    echo "Stopped"
fi

=========================================================================
= How to get the first line of a file in a bash script?=========================================================================

line=$(head -n 1 filename)

=========================================================================
= $var vs "$var"

=========================================================================

$ f="fafafda
> adffd
> adfadf
> adfafd
> afd"

$ echo $f
fafafda adffd adfadf adfafd afd

$ echo "$f"
fafafda
adffd
adfadf
adfafd
afd

=========================================================================
= Single line conditional statements
=========================================================================
if [ ! -f ./some_file ]; then
    print "The file was not copied"
    exit 1
fi
is equivalent to
[ ! -f ./some_file ] && print "The file was not copied." && exit 1


=========================================================================
= Tricky points
=========================================================================

------------------- Error: [: missing `]' ----------------------------------------------
#!/bin/sh

mkubifs_args="-m 4096 -e 253952 -c 2146 -F"
ubinize_args="-m 4096 -p 256KiB -s 4096"

#Error: [: missing `]'
if [ -z "$mkubifs_args"] || [ -z "$ubinize_args" ]; then
#if [ -z "$mkubifs_args" ] || [ -z "$ubinize_args" ]; then
    echo "MKUBIFS_ARGS and UBINIZE_ARGS have to be set, see http://www.linux-mtd.infradead.org/faq/ubifs.html for details"
fi