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#Bash Learning Notes

1 echo $? print the return state of last command, 0 success, 1 failed__

2 echo $PATH show default path__

3 which <command name or scrit name> show path of command/script__

4 Build in Command no need to load from disk, external command load from disk, build in command get use first if an external command has same name.

5 to validate if a command is build in command type <command name>

1 export VARNAME=name, define varibale which can be accessed outside the current shell

2 read A, read input from user and assign the value to A

3 a=$(command) e.g a=$(date)

1 Bash -x <script Name> to see the each steps of script running, + indicate code which is excuted, without + is the output of shell

1 $1 $2 refer to the 1st ,2nd input parameter
2 $? refer to all input parameters, used in for loop
3 $# count the input varibale number

1 -z $1 check if the first parameter is empty, return true if it is empyt, e.g [if -z $1 -eq 1]
2 [[-z $1]] && exit 1 '&&' only execute the 2nd command if the frist command is true
3 [[-z $1]] || exit 1 , || only execute the 2nd command if the frit command is false 4 [ -f $a ], check if $a is a file
5 [ -d $a ], check if $a is a directory
6 if [ $a -gt $b ] or if (( $a>$b )), -lt = <
7 [ -e $a ], check if file exist regardless if it is aregular file

To do subsitution for the lines in a file, use for loop e.g for i in $(cat <filename>) ; do <substitution>; done
1 echo ${DATA:-word} //return word if DATA not exist, but not set DATA
2 echo ${DATA:=word} //return word if DATA not exist and set DATA=word
3

  D=$(date)  
  echo $D  
  Thu 22 Mar 2018 18:28:48 GMT  
  echo ${D:0:2}  
  Th

4

MB17598:~ yu.an$ DATA=   
B17598:~ yu.an$ echo ${DATA:?Error}    
-bash: DATA: Error  

1 wall << a will boardcast all the followed command until it see string a

1 ${VAR#pattern} //serach for pattern form beginning of varibable's vaule, delete the shortest path that match, ## match the longest path and retrunt the left part

MB17598:~ yu.an$ Date=$(date)
MB17598:~ yu.an$ echo ${Date#*2}
4 Mar 2018 22:07:58 GMT
MB17598:~ yu.an$ echo ${Date##*2}
:07:58 GMT

2 ${VAR%pattern} search the pattern from the end of string

MB17598:~ yu.an$ echo ${Date%2*}  
Sat 24 Mar 2018 2  
MB17598:~ yu.an$ echo ${Date%%2*}  
Sat  

1 echo $((1+1))
2 bc deal with non-integers caculation echo "scale=9; 10/3" | bc

1 capture mutile matching , string1 or string B grep -e string1 -e string 2

1 cut -f 1 -d : <file> , -f 1 first filed, -d :, delimater is :
2 To cut string , use echo "abc def" | cut -f 2 -d ' '

awk -F <delimater> : '/search pattern/ {Action}' 1 awk -F : '{print $4}' /etc/passwd
2 awk -F : '/user/ {print $4}' /etc/passwd
3 awk -F : '$3>200' /etc/passwd , print line if the 3rd filed > 200
4 awk -F : '$NF ~ /bin/' /etc/passwd,print line if the last filed contatined bin
5 Use awk in string, echo "abc def" | awk '{ print $2 }'

1 echo hello | tr [a-z] [A-Z]
2 echo hello | tr [:lower:] [:upper:]

   if expression  
   then 
       command 
   elif 
   then  
      command 
   fi   

&& and , [ -z $1 ] && echo variable $1 not exist
|| or , `[ -z $1 ] && echo variable $1 not exist || not exist``

for i in something   
do   
	command1
	command2
done 

e.g

for i in {1..5}  
do   
	ping -c 1 192.168.0.$1  >/dev/null && echo 192.168.0.$ is pingable   
done    

case $var in  
<string1>)  
echo string1 ;;  
<string2>|<string3>)  
echo string2 or string 3 ;;  
*)    
echo other string 
;;  
esac 

while condition  
do  
	command  
done  

e.g

while [ $a> 4 ]; do  a=$(($a-1)) && echo $a; done

function help   
{  
 echo hi
}   

or

help ()  
{
echo hi  
}  

To include the functions from another script, use source /path/to/script

1 names=(linda lisa larua) names[3]=<name>, echo ${names[2]}
2 echo ${names[@]} show all elements
3 echo ${#names[@]} count the number of element

#! /usr/bin/env bash
echo select from below 
select DIR in /var /var/log /etc
do  
	case $REPLY in  
	1 ) cd /var/;;  	
	2 ) cd /var/log ;;  
	3 ) cd /etc ;;  
done  

note: use $REPLAY to refer to the unser input value;

trap "echo quit" INT INT is signal of Ctrl + C