10.9.4 “uniq” Command
This command performs the function of removing adjacent redundant data when reading data from the input or exporting the data to the output.
[Command Format]
uniq [option] [input] [output]
[Command Overview]
■ This removes adjacent redundant row data from input or output.
■ User privilege — Normal user.
10.9.3 “sort” Command
This command sorts the data in the specified file according to certain rule and then exports them to standard output.
[Command Format]
sort [option] [directory/file]
[Command Overview]
■ This sorts the data in the specified file.
■ User privilege — Normal user.
10.9.2 “wc” Command
This command counts and returns the number of line, word, and byte in the specified file.
[Command Format]
wc [option] [directory/file]
[Command Overview]
■ This calculates and reports the number of line, word, and byte in the specified file.
■ User privilege — Normal user.
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.
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.
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 “<" and ">” (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 << -- Sends the processing result of the right to the left. ■ > and >> — Sends the processing result of the left to the right.
■ < and > — Overwrite existing file if one exists
■ << and >> — Append existing file if one exists
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
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.
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.
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.