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.
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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.
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.
10.5.2 “test” Command – Logical Operation
This command checks the status of the file or performs a logical operation to specify exit status according to whether the result is “True” or “False”.
10.5 Commands Related to Operation
10.5.1 “expr” Command – Arithmetic Operation
You can use the “expr” command to perform arithmetic operations.
10.4.4 “alias” Command
On Linux, there is a feature that allows you to specify an alias name for a certain string in advance, and then use the defined alias name later instead of the string. The command to use at this time is the “alias” command.
10.4.3 “man” Command – Get Help on Commands
Most of the commands used in Linux can perform a variety of functions in a single command. In order to correctly execute these various functions provided by a specific command, the command must be input in various formats. Also, the execution result of the command is displayed in various types and various forms. In reality, for each command, we can hardly remember all necessary knowledge about what functions the command can do, and how to instruct the command, and how to understand the outcome of the execution.
So Linux provides detailed help for each command. With the help, users can search various informations that they can not remember when they need it, and solve the problem. The “man” command is used in this case.
10.4.2 “echo” Command
This command displays the specified text on standard output. It is not necessary for common task, but is often used to display information about various progresses when writing bash scripts.
10.4 Basic Command
10.4.1 “clear” Command – Clear Screen
When you work on the Shell Terminal, various contents are displayed on the screen. It can be inconvenient if the content of the screen is large or complex. In this case, you want to clear the current Terminal screen and start a new one. When you want to erase all the contents of the screen and start a new one, use the following command.
10.3.2 Applied Priority of Profile
When the system is started and a particular user logs on, what is applied to the system is the final result of the various profiles integrated according to their priority. By default, the local profile has priority over the global profile, and the application profile has a higher priority than the user profile.
The applied priority is that the above has a higher priority as shown below.