![example
#!/bin/bash
# alive2.sh
# Checks to see if hosts 192.168.0.1-192.168.0.20 are
alive
# Iterate through IP addresses
for n in {1..20}; do
host=192.168.0.$n
ping -c2 $host &> /dev/null
if [ $? = 0 ]; then
echo "$host is UP"
else
echo "$host is DOWN"
fi
done](https://image.slidesharecdn.com/unit7standardio-120309010645-phpapp02/85/Unit-7-standard-i-o-15-320.jpg)
Unit 7 standard i o
- 1. RedHat Enterprise Linux Essential Unit 7: Standard I/O and Pipes
- 2. Objectives Upon completion of this unit, you should be able to: Redirect I/O channels to files Connect commands using pipes Use the for loops to iterate over sets of values
- 3. Standard Input and Output Linux provides three I/O channels to Programs Standard input (STDIN) - keyboard by default (0) Standard output (STDOUT) - terminal window by default (1) Standard error (STDERR) - terminal window by default (2) ls –l myfile nofile
- 4. Redirecting Output to a File STDOUT and STDERR can be redirected to files: command operator filename Supported operators include: > Redirect STDOUT to file 2> Redirect STDERR to file &> Redirect all output to file File contents are overwritten by default. >> appends.
- 5. Redirecting Output to a File command > file - Direct standard output of command to file command >> file - Append standard output of command to file command < file - Send file as input to command command 2> file - Redirect error messages from command to file command 2>> file - Append error messages from command to file
- 6. Redirecting Output to a File Examples This command generates output and errors when run as non- root: $ find /etc -name passwd Operators can be used to store output and errors: $ find /etc -name passwd > find.out $ find /etc -name passwd 2> /dev/null $ find /etc -name passwd > find.out 2> find.err
- 7. Redirecting STDOUT to a Program (Piping) Pipes (the | character) can connect commands: command1 | command2 Sends STDOUT of command1 to STDIN of command2 instead of the screen. STDERR is not forwarded across pipes Used to combine the functionality of multiple tools command1 | command2 | command3... etc
- 8. Redirecting STDOUT to a Program Examples less: View input one page at a time: $ ls -l /etc | less Input can be searched with / mail: Send input via email: $ echo "test email" | mail -s "test" student
- 9. Combining Output and Errors Some operators affect both STDOUT and STDERR &>: Redirects all output: $ find /etc -name passwd &> find.all 2>&1: Redirects STDERR to STDOUT Useful for sending all output through a pipe $ find /etc -name passwd 2>&1 | less (): Combines STDOUTs of multiple programs $ ( cal 2007 ; cal 2008 ) | less
- 10. Redirecting to Multiple Targets (tee) $ command1 | tee filename | command2 Stores STDOUT of command1 in filename, then pipes to command2 Uses: Troubleshooting complex pipelines Simultaneous viewing and logging of output ls -lR /etc | tee stage1.out | sort | tee stage2.out | uniq -c | tee stage3.out | sort -r | tee stage4.out | less
- 11. Redirecting STDIN from a File Redirect standard input with < Some commands can accept data redirected to STDIN from a file: $ tr 'A-Z' 'a-z' < .bash_profile This command will translate the uppercase characters in .bash_profile to lowercase Equivalent to: $ cat .bash_profile | tr 'A-Z' 'a-z'
- 12. Sending Multiple Lines to STDIN Redirect multiple lines from keyboard to STDIN with <<WORD All text until WORD is sent to STDIN Sometimes called a heretext $ mail -s "Please see" student@localhost <<END > Hi all, >I’m Tam > END
- 13. Scripting: for loops Performs actions on each member of a set of values Example: for NAME in joe jane julie do ADDRESS="[email protected]" MESSAGE='Projects are due today!' echo $MESSAGE | mail -s Reminder $ADDRESS done
- 14. Scripting: for loops continued Can also use command-output and file lists: for num in $(seq 1 10) • Assigns 1-10 to $num • seq X Y prints the numbers X through Y for file in *.txt • Assigns names of text files to $file
- 15. example #!/bin/bash # alive2.sh # Checks to see if hosts 192.168.0.1-192.168.0.20 are alive # Iterate through IP addresses for n in {1..20}; do host=192.168.0.$n ping -c2 $host &> /dev/null if [ $? = 0 ]; then echo "$host is UP" else echo "$host is DOWN" fi done