All about redirection

to file

stderr to stdout

stderr and stdout 2 file

stdout to stderr

ls -l > ls-l.txt

ls not_exist_file 2> &1

grep for 1>&2

rm -f $(find / -name core_hello) &> /dev/null 

Pipes

simple pipe with sed

 ls -l | sed -e "s/[aeio]/u/g"   

an alternative to ls -l *.txt

ls -l | grep "\.txt$"

Conditionals

Dry Theory

 if expression then statement1 else statement2

 if expression1 then statement1 else if expression2 then statement2 else statement3

Conditionals with variables

T1="foo"
T2="bar"
if [ "$T1" = "$T2" ]; then
    echo expression evaluated as true
else
    echo expression evaluated as false
fi

Variable

using variables

STR="Hello World!"
echo $STR 

simple backup script

OF=/var/my-backup-$(date +%Y%m%d).tgz
tar -zcvf $OF /home/me/

HELLO=Hello 
function hello {
        local HELLO=World
        echo $HELLO
}
echo $HELLO
hello
echo $HELLO

local variables

Loops for, while and until

Numeric ranges

for i in 1 2 3 4 5
do
   echo "Welcome $i times"
done

bash v3.0+ has inbuilt support for setting up ranges

for i in {1..5}
do
   echo "Welcome $i times"
done

echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
  do
     echo "Welcome $i times"
 done

Bash v4.0+ support {START..END..INCREMENT} 

Three expression bash for loops syntax

for (( c=1; c<=5; c++ ))
do
   echo "Welcome $c times"
done
 

Conditional exit with break

for I in 1 2 3 4 5
do
  statements1     
  statements2
  if (disaster-condition)
  then
	break       	   #Abandon the loop.
  fi
  statements3          #While good and, no disaster-condition.
done

While example

COUNTER=0
 while [  $COUNTER -lt 10 ]; do
 echo The counter is $COUNTER
 let COUNTER=COUNTER+1 
 done

Until example

 COUNTER=20
 until [  $COUNTER -lt 10 ]; do
     echo COUNTER $COUNTER
     let COUNTER-=1
 done

Functions

Functions sample

function hello {
   echo Hello!
}
hello

Functions with parameters

function e {
    echo $1 
}  
e Hello

Comparison Operators

integer comparison

-lt (<)
-gt (>)
-le (<=)
-ge (>=)
-eq (==)
-ne (!=)

string comparison

s1 = s2
s1 != s2
s1 < s2
s1 > s2

find number which can be divided by 7

for i in {1..100..1};
 do let j=$i%7 ;
 if [ $j -eq 0 ] ; 
 then echo $i;
 fi;  
done;  

Userful command

wc (counts lines, words and bytes)

wc -w -l -c vimtest.txt

grep (print lines matching a search pattern)

grep "hello" vimtest.txt -c

sed (stream editor)

sed 's/to_be_replaced/replaced/g' vimtest.txt
sed '2,5d' vimtest.txt

User interface

Using select to make simple menus

   #!/bin/bash
   OPTIONS="Hello Quit"
   select opt in $OPTIONS; do
       if [ "$opt" = "Quit" ]; then
        echo done
        exit
       elif [ "$opt" = "Hello" ]; then
        echo Hello World
       else
        clear
        echo bad option
       fi
   done
  

Test if the program has received an argument 

#!/bin/bash   
if [ -z "$1" ]; then 
      echo usage: $0 directory
      exit
 fi    

Misc

Reading user input with read

read NAME
echo "Hi $NAME!"

Arithmetic evaluation

echo $((1+1))
echo $[1+1]

Getting the return value

$?

Debugging

Ways Calling BASH

    
 #!/bin/bash 
 set -ex