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Output redirection is more commonly used than input redirection. Output redirection enables you to redirect the output from a command into a file, as opposed to having the output displayed on the screen.
There are many situations in which this capability can be very useful. For example, if the output of a command is quite large and does not fit on the screen, you may want to redirect it to a file so you can view it later using a text editor. Output redirection is done in much the same way as input redirection. Instead of using the < symbol, the > symbol is used.
To redirect the output of an ls command into a file named directory.out, the following command is used:
ls > directory.out
Pipelines are a way to string together a series of commands. This means that the output from the first command in the pipeline is used as the input to the second command. You can tell pdksh to create a pipeline by typing two or more commands separated by the | character. The following is an example of using a pdksh pipeline:
cat test.file | sort | uniq
This is a fairly common pipeline. Here, the contents of test.file (the output from the cat command) are fed into the input of the sort command. The sort command, without any options, sorts its input alphabetically by the first field in the input. The sorted file is then piped into the uniq command. The uniq command removes any duplicate lines from the input. If test.file contains the lines
Sample dialog Hello there How are you today Hello there I am fine
the output from the pipeline is the following:
Hello there How are you today I am fine Sample dialog
All of the lines in the file have been sorted by the first word in the line, and one of the Hello there lines has been removed because of the uniq command.
pdksh has three levels of user prompts. The first level is what the user sees when the shell is waiting for a command to be typed. (This is what you normally see when you are working with the shell.) The default prompt is the $ character. If you do not like the dollar sign as the prompt or prefer to customize the prompt, you can do so by setting the value of the PS1 pdksh variable.
To set a variable, give the name and equal sign, and the string you want to set it to. Make sure you do not place any spaces on either side of the equal sign or the shell will not interpret your command properly. For example, the line
PS1=“! Tell me what to do”
sets the shell prompt to the string ! Tell me what to do. The pdksh shell keeps track of how many commands have been entered since it was started. This number is stored into the shell variable called !. When you include the ! in the prompt, it displays the current command number in the prompt. The previous prompt command causes the command number followed by the string Tell me what to do to be displayed on the command line each time pdksh is expecting you to enter a command.
The second level prompt is displayed when pdksh is expecting more input from you in order to complete a command. The default for the second level prompt is >. If you want to change the second level prompt, you can do so by setting the value of the PS2 pdksh variable, as in the following example:
PS2=“ I need more information”
This causes the string I need more information to be displayed on the command line whenever pdksh needs something from you to complete a command.
pdksh does not support the advanced prompt options that bash supports. There is no predefined set of escape codes that you can put in a pdksh prompt variable to display such items as the time or current working directory. You can, however, put other pdksh variables into a prompt variable. For example, the following two prompts are valid:
PS1=“(LOGNAME) ” PS1=‘($PW(D) ’
The first example causes your prompt to be equal to your UNIX username. The second example causes your prompt to be the current working directory. The single quotes are needed here so that the value of the PWD variable does not get assigned to the variable only the first time it is executed. If you use double quotes, the PWD variable is evaluated only when the command is first entered. (The prompt will always be the directory name of the directory that you are in when you enter the command.) The single quotes cause the value of the PS1 variable to be equal to the current value of the PWD variable. For more information on using these quotes, see Chapter 14, “Shell Programming.”
Job control is the capability to control the execution behavior of a currently running process. Specifically, you can suspend a running process and cause it to resume running at a later time. The pdksh shell keeps track of all of the processes that it starts, and you can suspend a running process or restart a suspended one at any time during the life of that process.
Pressing the Ctrl+Z key sequence suspends a running process. The bg command restarts a suspended process in the background and the fg command restarts a process in the foreground.
These commands are most often used when a user wants to run a command in the background but accidentally starts it in the foreground. When a command is started in the foreground, it locks the shell from any further user interaction until the command completes execution. This is usually not a problem because most commands take only a few seconds to execute. If the command you are running is going to take a long time, you typically start the command in the background so that you can continue to use pdksh to enter other commands while it completes running.
If you start a command in the foreground that is going to take a long time, your shell may be tied up for several minutes. If you have done this and want to continue executing the command in the background, enter the following:
control-z bg
This suspends the command and restarts it in the background. The command continues to execute and you have control of pdksh.
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