lec5vid1&2AdvanceIO

#!/bin/bash

# This script demonstrates I/O redirection
#EACH NEW PROCESS OPENS WITH THREE NEW OPEN FILE DESCRIPTERS , FD 0 - STANDARD INPUT , FD 1 - STANDARD OUTPUT , FD 2 - STANDARD ERROR 

# Redirect STDOUT to a file 
FILE='//home/devfedora/Debankan/bash-scripting/lec5vid1'
head -n2 /etc/passwd > ${FILE} # output generated in the file use cat to view 
cat $FILE
echo '1'

# Redirect STDIN to a program
FILE="/home/devfedora/Debankan/bash-scripting/cities"
read LINE < ${FILE}
echo "The line contains: ${LINE}"
echo '2'

# Redirect STDOUT to a file , overwriting the file 
FILE='//home/devfedora/Debankan/bash-scripting/lec5vid1'
tail -n3 /etc/passwd > ${FILE}
cat $FILE
echo '3'

# Redirect STDOUT to a file , appending to the file 
echo "$RANDOM $RANDOM" >> ${FILE}
cat $FILE
echo '4'

#--- VIDEO 2---
#Redirecting standard input to a program, using FD 0
read LINE 0< ${FILE}
echo $LINE
echo '5'

# Redirecting standard error to a file 
head -n1 /etc/passwd /etc/hosts /fakefile 2> head.err # we use 0 for standard input file descriptors, 1 for standard output file descriptors, 2 for standard error output file descriptors
cat head.err
echo '6'

#Redirecting standard output and standard error output both to a file 
head -n1 /etc/passwd /etc/hosts /fakefile > head.both 2>&1 #here output is generating in head.both which is 1 and standard error output which is 2 is appending with standard output file descriptors WE CAN ALSO USE: head -n1 /etc/passwd /etc/hosts /fakefile &> head.both
cat head.both
echo '7'

# Pipe works as standard input but standard error doesnot flows through pipe 
head -n1 /etc/passwd /etc/hosts /fakefile | cat -n

# if you want pipe to catch the standard error along with standard output 
head -n1 /etc/passwd /etc/hosts /fakefile 2>&1 | cat -n #SHORTCUT: head -n1 /etc/passwd /etc/hosts /fakefile |& cat -n

# Send output to standard error
echo "THIS IS STDERR" >&2

# Discard STDOUT
echo
echo "discarding STDOUT:"
head -n3 /etc/passwd /fakefile > /dev/null

# Discard STDERR
echo
echo "discarding STDERR:"
head -n3 /etc/passwd /fakefile 2> /dev/null

# Discard STDOUT AND STDERR
echo
echo "discarding STDOUT AND STDERR:"
head -n3 /etc/passwd /fakefile &> /dev/null

 # clean up
 rm $FILE &> /dev/null