Raspberry Pi_Eng_10.9.1 “awk” Command

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10.9 Commands Related to Data Manipulation

10.9.1 “awk” Command

The “awk” command reads the specified file, finds the line that has a value matching the specified pattern, and performs the specified operation if the pattern matches. Here, the operation means a field manipulation in a line or an arithmetic operation using a field value. The name “awk” comes from the names of three people who developed it, Aho, Weinberger, and Kernighan.

Awk is a programming language with features such as Shell programming, BC, and the C programming language. It works perfectly with BC, and field variables with names like Shell arguments $1, $2, and $3 can be used on each input line. It also has a printing and control operator similar to the C language.

Raspberry Pi_Eng_10.8.3 | (pipe) Command

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10.8.3 | (pipe) Command

In Linux, there is a way to use the output from the execution of one command as input to another command, which is “|” (pipe) command. The word “pipe” here means that all incoming input to the pipe is not removed or sent elsewhere, but is passed to the exit as it is entered without loss of content.

Normally, when a command is executed, it receives input data from an explicitly specified file or input device, processes it in the manner specified by the command, and sends the result to the output.

Raspberry Pi_Eng_10.8.2 Redirection Command – Change Input and Output Device

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10.8.2 Redirection Command – Change Input and Output Device

In Linux, when you run a particular command, it sends its execution results to a predefined output device. However, if necessary, the execution result can be sent to an output device other than the predefined output device.

It is “” (redirection) that is used in this case. If you use this command, it sends the execution result to the output device specified after this command. “redirect” means to change the default input device and the default output device where are set as the keyboard and monitor to another device.

Note that there are several commands as follows for user to redirect input and output with:

■ < and < and >> — Sends the processing result of the left to the right.

■ — Overwrite existing file if one exists

■ <> — Append existing file if one exists

Raspberry Pi_Eng_10.8.1 “xargs” Command

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10.8 Commands to Adjust Input and Output

10.8.1 “xargs” Command

This command allows you to use the output of the previously executed command as an argument input to the next command.

[Command Format]

xargs [option] exec-command

Raspberry Pi_Eng_10.7.4 “exit” Command – Terminate Process

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10.7.4 “exit” Command – Terminate Process

The “exit” command is used to terminate process while a process is in progress.

[Command Format]

exit [exit-status]

[Command Overview]

■ This terminates the running process and set the exit status to the specified value.

■ User privilege — Normal user.

Raspberry Pi_Eng_10.7.3 Executing Program on Background/Foreground

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10.7.3 Executing Program on Background/Foreground

Normally when we execute a command, after one command is completed, the next command is executed. We call this method of processing the foreground mode. In this way, another command can not be entered while you are working on one operation. The Raspberry Pi system always executes the command in foreground mode unless any special instruction is not issued.

Some of the tasks that you perform on the system may end in a short time, but there may be work that takes quite a while. If you have a job that takes a long time, you can start another job after it finishes, but if the next job to process is not related to the current processing job, it is possible to start the next job immediately without waiting for the current processing job to finish.

Raspberry Pi_Eng_10.7.2 “;” Command – Execute Multiple Commands Successively

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10.7.2 “;” Command – Execute Multiple Commands Successively

Linux provides a way to enter and process all the commands at once, if you have multiple commands to process. To execute several commands at once in this way, link each command with the “;” command as follows.

Raspberry Pi_Eng_10.7.1 “which” Command – Check Path Name of Executable File

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10.7 Commands Related to Program Execution

10.7.1 “which” Command – Check Path Name of Executable File

The “which” command tells you what program is actually being used when executing certain command on the system. In fact, many programs are installed in many places in the system, so it is often difficult to judge which program is actually used. The “which” command is used in this case.

Raspberry Pi_Eng_10.6.2 “||” Command – Execute Command by “or” Method

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Original Book Contents
10.6.2 “||” Command – Execute Command by “or” Method

This command processes the next command when the first command does not processed normally, and sets the exit status by “or” operation according to the final processing result of the both commands.

Raspberry Pi_Eng_10.6.1 “&&” Command – Execute Command by “and” Method

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10.6 Commands Related to Selective Execution

Normally, when a command is processed normally, the exit status is set to zero, which is ㅅtreated as “True”. On the other hand, if the command is not processed normally, the exit status is set to a non-zero value, which is treated as “False”.

10.6.1 “&&” Command – Execute Command by “and” Method

This command processes the next command if the first command is processed normally, and sets the exit status by “and” operation according to the final processing result of the both commands.