Fundamentals of Unix Tutorial
PIPE Commands and REDIRECTION Commands – CH 7
The content of this tutorial also applies to linux.
Piping
UNIX supports and makes extensive use of the ability to connect
commands to one another. By piping commands together, you can use the
output of one command as the input for another.
The pipe symbol is | (a vertical bar). To pipe commands together you
simply place a pipe between each command on the command line. For
example
command1 | command2 | command3
Although you can pipe many commands together, you'll rarely need to do
so.
Examples of using pipes to connect commands
Here are some examples you can learn from and adapt for your own uses.
If you don't completely understand the commands are that are being piped,
look them up in other parts of this tutorial or in the glossary of the CiscoSun online curriculum.
Example: ls -alFs | more
This command sends the output of the ls command with the options alFs
to the more command. The options used with ls indicate that all files,
including hidden ones will be listed, in long form, marking directories
with a slash (/), executables (programs) with an asterisk (*), and the size
of each file will be listed as well. Normally such a list would scroll by too
quickly to view, but by piping the output through the more command you
can view one page at a time.
Example: who | wc -l
This command takes the output from the who command which lists
everyone currently logged in to the system and passes the information to
the wc command which, using the -l option, counts the number of lines
that the who command generated. Since who generates a line for each
person logged in, the number of lines is equal to the number of people
logged in to the system.
Example: who | grep Chris
Similar to the first example, this command pipes the output of the who
command to the grep command which searches for instances of the letters
"Chris". If grep finds a line containing the letters, grep prints the line(s) to
the screen. This command would tell you how many people with the user
name Chris are logged in.
Example: Connecting several commands together:
ps –ef | grep joe | sort +5 –6 | less
The first command, ps –ef, outputs information about the processes currently
running. This information becomes the input to the command grep joe which
searches for any lines that contain the username "joe". The output from this
command is then sorted on the sixth field of each line, with the output being
displayed one screenful at a time using the less pager (similar to more).
Redirection
Normally everything you do on the keyboard is sent to the computer
screen for your viewing pleasure. That is, when you type a command, the
letters you type are sent to the screen so you can see what you're typing.
The results of the command are also sent to the screen so you can see what
happened.
Your typing on the keyboard is the "input" and what you see on the screen
is the "output." To UNIX, a computer screen and a file are forms of
output. The screen is the standard output, normally abbreviated as
"stdout." With redirection you can send output to files instead of the
screen, or you can use what's in a file as the input for a command. With
redirection you can change where the computer gets input, and where it
puts output.
You use the symbols < , > , >> , and 2> to redirect input and output.
Examples of using redirection
Redirecting standard output: To redirect the standard output from a
command use the > (greater than) symbol followed by the name of the
output file. If the file that you redirect standard output to does not already
exist it will be created.
Example: ls -la > mydir
This command lists all the files and directories in long form, and sends
them to the file mydir. If the file mydir does not exist, it will be created.
If it does exist it will be overwritten with the new data. When you execute
this command, you will not see anything appear on the screen (unless
there is an error – that is described below).
Example: grep Smith /etc/passwd > popular
This redirects the standard output from the grep command so that it goes
to the file popular. This file will contain all the lines containing
occurrences of the string “Smith” that were found in the /etc/passwd file.
Example: cat section1 section2 section3 > Chapter1
This example connects (concatenates) several files in order, and outputs them as
one file, Chapter1. The cat command reads files section1 through section3 in
turn and appends them to (tacks them onto the end of) the file Chapter1. The
contents of section2 are appended to section1 and so on.
Example: who > people
This prints the list of active users to the file people.
Example: sort file1 > sorted_list
This sorts the contents of file1 and saves the sorted list as sorted_list.
Appending standard output: To append the standard output from a command to
a file, use two >> (greater than) symbols followed by the name of the file. If the
file does not exist it is created. For example:
Example: ls >> mydir
This appends the data from a short form directory listing to the data (if
any) already in the file mydir.
Example: If you wanted to add one more section to the file Chapter1,
created earlier, you'd append the text using the command
cat section4 >> Chapter1
to make sure the text that already exists in the file isn't disturbed.
Redirecting standard input: Redirecting input is just as easy. For
example, say you want to mail someone the directory list you just made,
called mydir. Instead of having to import it into a mail program, you can
redirect the input for the mail command:
Example: mail Tony < mydir
This command uses the contents of the mydir file as the input for the mail
command. The effect of this is to mail the contents of this file to the user
Tony.
Redirecting error output: Even though you send the standard output to a
file, if an error occurs the error message will be sent to your screen. This is
because the standard error output, or “stderr”, is the screen. If you wish to
redirect error messages to a file for troubleshooting purposes, use the 2>
symbol. For example:
cat file1 file2 file3 2> Error_Log
Any errors that occur during the execution of this command will be
included in Error_Log. Since we are troubleshooting, the output, if any,
of the command will probably be incorrect. If there is any output, it will be
directed to the screen, since we did not direct it to a file. We directed only
the error messages to a file so we could have a record of them.
Example of combining piping and redirection
You can combine piping with redirection to create files that contain the
output.
Example: who | grep Chris > List_of_Chris
In the example above, the list of lines from the who command containing
the letters "Chris" is redirected from the screen (the standard output
device) to the file List_of_Chris. You could view the resulting text file
with a viewer such as the more command, or an editor such TED or
Nedit, or the text editor in the Solaris CDE.