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Using redirection and pipes

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Text files processing Using redirection and pipes Use grep and extended regular expressions to analyze text files Connecting to the server via ssh, using multiple consoles and screen command Introduction to Unix permissions model Tips Sysadmin Horror Stories Unix History with some Emphasis on Scripting Humor

On Red Hat systems, users store their files in the /home/USERNAME directory. root has a special non-mounted home directory at /root  

I/O Redirection

By default when a command is executed it shows its results on the screen of the computer you are working on. The computer monitor is the so-called standard output, which is also referred to as the STDOUT. The shell also has default destinations to send error messages  (STDERR) to and to accept input (STDIN)

So if you run a command, that command would expect input from the keyboard, and it would normally send its output to the monitor of your computer without making a difference between normal output and errors. Some commands, however, are started at the background and not from a current terminal session, so these commands do not have a monitor or console session to send their output to, and they do not listen to keyboard input to accept their standard input. That is where redirection comes in handy.

Programs started from the command line have no idea what they are reading from or writing to. They just read from file descriptor 0 if they want to read from standard input, and they write to file descriptor number 1 to display output and to file descriptor 2 if they have error messages to be output. By default, these are connected to the keyboard and the screen.

If you use redirection symbols such as <, >, and |, the shell connects the file descriptors to files or other commands.

In I/O redirection, files can be used to replace the default STDIN, STDOUT, and STDERR. You can also redirect to device files. A device file on Linux is a file that is used to access specific hardware. Your hard disk for instance can be referred to as /dev/sda, the console of your server is known as /dev/console or /dev/tty1, and if you want to discard a commands output, you can redirect to /dev/null. To access most device files you need to be root.

Redirection and pipes

There are two major mechanism that increase flexibility of Unix utilities:

Before shell executes a command, it scans the command line for redirection characters. These special symbols instruct the shell to redirect input and output accordingly. Redirection characters can appear anywhere in a simple command or can precede or follow a command. They are not passed on to the invoked command as parameters.

Redirection basics

By default Unix/Linux assumes that all output is going to STDOUT  which is assigned to a user screen/console called  /dev/tty. You can divert messages directed to standard output, for example from commands like echo,  to files or other commands. Bash refers to this as redirection.

The most popular is > operator, which redirects STDOUT to a file. The redirection operator is followed by the name of the file the messages should be written to. For example, to write the message "The processing is complete" to a file named my.log , you use

echo "The processing started at $timestamp" > /tmp/my.log

Try to execute

echo "Hello world" > /dev/tty

You will see that it typed on the your screen exactly the say way as if you executed the command

echo "Hello to myself"

Because those two command are actually identical.

It is important to understand that when messages aren't redirected in your program, the output goes through a special file called standard output. By default, standard output represents the screen. That means that everything sent through standard output is redirected to the screen. Bash uses the symbol &l to refer to standard output, and you can explicitly redirect messages to it. You can redirect to the file the output of the whole script

bash > mylisting.txt
This is the same as
bash 1> mylisting.txt

In this case any echo statement will write the information not the  screen, but to the file you've redirected the output to. In this case this is the file mylisting. txt

But you can also redirect each echo statement in you script. Let's see another set of examples:

echo "Don't forget to backup your data" > /dev/tty      # send explicitly to the screen
echo "Don't forget to backup your data"                 # sent to screen via standard output stream
echo "Don't forget to backup your data >&1              # same as the last one
echo "Don't forget to backup your data >/dev/stdout     # same as the last one
echo "Don't forget to backup your data" > warning.txt   # redirect to a file in the current directory

Using standard output is a way to send all the output from a script and any commands in it to a new destination.

A script doesn't usually need to know where the messages are going: There’s always the possibility they were redirected. However, when errors occur and when warning messages are printed to the user, you don't want these messages to get redirected along with everything else.

Linux defines a second file especially for messages intended for the user called standard error. This file represents the destination for all error messages. Because standard error, like standard output, is a file, standard error can likewise be redirected. The symbol for standard error is &2.  Instead of &2 /dev/stderr can also be used. The default destination for standard error, like standard output, is the screen. For example,

echo "$SCRIPT:SLINENO: No files available for processing" >&2

This command appears to work the same as a echo without the >&2 redirection, but there is an important difference. It displays an error message to the screen, no matter where standard output has been previously redirected.

 The redirection for the standard error is very similar but naturally they begin with the number 2. For example

bash 2> myscript_errors.txt

You can merge standard output and standard error streams with  2>&1: This redirects the stderr to the stdout, which in turn can be redirected to a file:

bash 2>&1 > myscript_errors.txt

There are several classic types of redirection. Among them: 

Source and target can be expression. In this case bash performs command and parameter substitution before using the  parameter. File name substitution occurs only if the pattern matches a single file

Unix command cat is actually short for "catenate," i.e., link together. It accepts multiple filename arguments and copies them to the standard output. But let's pretend, for the moment, that cat and other utilities don't accept filename arguments and accept only standard input. Unix shell lets you redirect standard input so that it comes from a file. The notation command <  filename does the same as cat with less overhead.

The > operator always overwrites the named file. If a series of printf messages are redirected to the same file, only the last message appears.

To add messages to a file without overwriting the earlier ones, Bash has an append operator, >>. This operator redirects messages to the end of a file.

echo "The processing started at $timestamp" > /tmp/my.log
... ... ... 
echo "There were no errors. Normal exist of the program" >>  /tmp/my.log

In the same way, input can be redirected to a command from a file. The input redirection symbol is <. For example, the utility wc (word count) is able to calculate number of lines in the file with the option -l. That means that you can count the number of lines in a file, using the command:

wc  -l <  $HOME/.bashrc

Again, wc -l count lines of the file. In this case this is number of lines in your  .bashrc. Printing this information from your .bash_profile script might be a useful reminder to you that can alert you to the fact that you recently modified your env, or God forbid your .bashrc file disappeared without trace :-)  

There is also a possibility to imitate reading from a file inside the script by putting several lines directly into the script. The operator <<MARKER treats the lines following it in a script as if they were typed from the keyboard until it reaches the file starting from the work MARKET. In other words the lines which are treated as an input file are limited by the a special line using the delimiter you you define yourself. For example, in the following example the delimiter word used is "EOL": 

cat > /tmp/example <<EOF
this is a test demostrating how you can 
write several lines of text into 
a file

If you use >> instead of  >  you can add lines to a file without using any editor:

cat >>/etc/resolv.conf <<EOF

In this example bash treats the three lines between the EOF markers as if they were being typed from the keyboard and write them to the file specified after > (/tmp/example in our case).  there should be no spaces between << and EOF marker. Again, the name EOF is arbitrary. you can choose, for example,  LINES_END instead. the only important thing is there should be no lines in your test that start with the same word.

cat >>/etc/resolv.conf <<LINES_END

There should no market at any beginning of the lines of included text. that's why using all caps makes sense in this case. 

The data in the << list is known as a here file (or a here document) because the word HERE was often used in Bourne shell scripts as the marker of the end of the input lines.

Bash have another here file redirection operator, <<<, which redirects a variable or a literal.

cat > /tmp/example <<<  "this is another example of piping info into the file" 
Here is a summary of what we can do.

Exercise: Using I/O Redirection

1. Open a shell as user user and type cd without any arguments. This ensures that the home directory of this user is the current directory while working on this exercise. Type pwd to verify this.

2. Type ls. You’ll see the results onscreen.

3. Type ls > /dev/null. This redirects the STDOUT to the null device, with the result that you will not see it.

4. Type ls /root/etc  > /dev/null. This command shows a “no such file or directory” message onscreen. You see the message because it is not STDOUT, but an error message that is written to STDERR.

5. Type ls /root/etc 2> /dev/null. Now you will not see the error message anymore.

6. Type ls /root/bin  /root/etc  2> /dev/null. This command will show content of /root/bin directory supressing all error message from non existant /root/etc directory

7. Type ls /root/bin  /root/etc  2> /dev/null > output. In this command, you still write the error message to /dev/null while sending STDOUT to a file with the name output that will be created in your home directory.

8. Type cat output to show the contents of this file.

9. Type echo hello > output. This overwrites the contents of the output file.

10. Type ls >> output. This appends the result of the ls command to the output file.

11. Type ls -R /. This shows a long list of files and folders scrolling over your computer monitor. (You may want to type Ctrl+C to stop [or wait a long time]).

12. Type ls -R | less. This shows the same result, but in the pager less, where you can scroll up and down using the arrow keys on your keyboard.

13. Type q to close less. This will also end the ls program.

14. Type ls > /dev/tty1. This gives an error message because you are executing the command as an ordinary user (unless you were logged in to tty1). Only the user root has permission to write to device files directly.

Pipes as cascading redirection

Linux administrator needs to know well how to use pipes, because pipes are used for construction simple sysadmin scripts on daily basis. Pipes were one of the most significant innovation brought to the OS area by Unix. By combining multiple commands using pipes, you can create kind of super commands that make almost anything possible. Pipe can be used to catch the output of one command and process it as input by  a second command. And so on.

Many text utilities are used as stages in multistage pipes (utilities which accept standard input processes it and output the result into standard output are called filters)

Pipeline programming involves applying special style of componentization  that allows to break a problem into a number of small steps, each of which can then be performed by a simple program. We will call this type of componentization pipethink in which wherever possible, programmer  relies on preexisting collection of useful "stages" implemented by Unix filters. David Korn quote catches the essence of pipethink -- "reuse of a set of components rather than on building monolithic applications".

This process is called piping and shell uses the vertical bar (or pipe) operator | to specify it:

who | wc -l # count the number or users

Any number of commands can be strung together with vertical bar symbols. A group of such commands is called a pipeline. This is actually a language that system administrators are learning all their carrier.  Level of mastery of this language directly correlates with the qualification of sysadmin.  See also Pipes -- powerful and elegant programming paradigm

Pipes are often used for processing log -- for example analyzing if there are some types of errors or selecting appropriate fragment for further analysis. For example to select lines for 100 to 200 you can use two stage pipe:

cat /etc/log/messages | head -200 | tail -100

If one command ends prematurely in a series of pipe commands, for example, because you interrupted a command with a Ctrl-C, Bash displays the message "Broken Pipe" on the screen.

 If a user runs the command ls, for instance, the output of the command is shown onscreen. If the user uses ls | less, the commands ls and less are started in parallel. The standard output of the ls command is connected to the standard input of less. Everything that ls writes to the standard output will become available for read from standard input in less. The result is that the output of ls is shown in a pager, where the user can browse up and down through the results easily.

This way less can serve as interactive frontend to any utilities that does not have such capabilities.

You can you filter pv (pipe viewer) to debug each pipe stage on different datasets. pv outputs on console each record that is passing via it.

You can also split pipe into two parts. One that is working properly and the other that is not. Write output of the first stage after the one that is working properly  into the file in /tmp and analyze it visually with less and other tools. This way you can find out what's wrong with the output of problematic stage, if any. After that you can add the next stage and repeat this procedure.  And so on until the whole pipe is debugged. 

You can use tee on any stage of the pipe to divert output to a file.


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