IT 1100 : Introduction to Operating Systems
As you read the section on the
cat command, please note that our primary usage of this command in this class is to display the content of a file. It does a whole lot more, but we won’t worry about that right now.
cat command has similar results to the
less command. It allows us to view the contents of a file. Try the following commands to see the difference.
- Linux programs can receive input known as stdin or standard input (primarily this comes from the keyboard)
- Programs have 2 types of output (standard output or stdout and standard error or stderr). stdout are normal output messages of a program (ie. the output generated by
ls -l) stderr are the status messages or error messages of a program (we will see these later)
- By default the 2 types of output are sent to the screen.
Sometimes you want to redirect the standard output from the screen to a file or some other location (a printer?). You can do this by using either
>> after the command.
ls -l > file_list.txt
echo "Who was in the movie Black sheep?" >> chris_farley.txt
Practice the above commands a few times and then use the
cat command to see if you can figure out the difference between the
Issue a command that generates an error like:
ls -l /var/i/dont/exist
Now , redirect that error to a file:
ls -l /var/i/dont/exist 2> bad_error
To redirect both stdout and stderr to same file do:
ls -l /var &> both.txt
Redirect output to the black hole
What if I don’t want to see output on the screen AND I don’t want to send it to a file? There is a location in linux to do that.
ls -l /var/ &> /dev/null
You can take the output of one command and send it to another command by using the pipe
| operator. Here are some examples:
ls -l | wc -l
The above command took the output of the ls command and piped it to the wc command. You could use
man wc to figure out what either command does if you don’t remember.
What does this command do?
echo "My name is fred" | wc
cat /etc/passwd | less
Read and Complete the Exercises in the Book
Last Updated 01/22/2018