Table of Content
Weight: 4
Description: Candidates should be able to redirect streams and connect them in order to efficiently process textual data. Tasks include redirecting standard input, standard output and standard error, piping the output of one command to the input of another command, using the output of one command as arguments to another command and sending output to both stdout and a file.
Key Knowledge Areas:
- Redirecting standard input, standard output and standard error
- Pipe the output of one command to the input of another command
- Use the output of one command as arguments to another command
- Send output to both stdout and a file
Terms and Utilities:
tee xargs
standard I/O stream
stdin 0 stdout 1 stderr 2 > create new file if not exist; o/w overwrite >> append exist file; o/w create new file 2> write stderr if exist; o/w overwrite it 2>> append stderr to exist file; o/w create new file &> stdout+stderr to exist file; o/w create new file same cmd &> file cmd >& file cmd > file 2>&1 ls x* z* &> output # stderr+stdout go to output ls x* z* >output 2>&1 # stderr+stdout go to output ls x* z* 2>&1 >output # stderr not go to output ls x* z* 2> /dev/null sort -k2 1 apple > 2 pear > 3 banana > EOF 1 apple 3 banana 2 pear gunzip -c file.tar.bz2 | tar -xvf - tar -vcf pam.tar /etc/sysconfig >/dev/null 2>&1 updatedb >& /dev/null & killall -HUP inetd sendmail &> /dev/null tar -vcf pam.tar /etc/sysconfig/ >debug 2>debug
tee
print out on screen stdout also to log file script |tee log scripttee -a log
find, xargs
find ./ -name "*~" |xarg -d "\n" rm # remedy for filename with space
rm find ./ -name "*~"
Quiz questions
1. What are the two methods to capture the output of a process and include it on the command line of another process? 2. How would you send the output of a process to a file named “output” and the errors to a file named “errors”? 3. What would you type in a script to have the standard input of the command defined in the script? 4. How do you send the output of one process as the input of another? 5. How do you send both standard output and standard error output to another program as its standard input? 6. How would you use tee to capture the intermediate results of find /etc before sorting them with sort? (Are there any differences between the unsorted and the sorted versions?)
Answers to quiz questions
1. a) $(process) and process b) xargs
2. process > output 2> errors
3. echo “input” | process; ... or ... process &1 | another-process
6. find /etc | tee unsorted | sort > sorted