Below is the file system we are referring to.
Home
└── lecture1
├── Hello.class
├── Hello.java
├── messages
│ ├── en-us.txt
│ ├── es-mx.txt
│ └── zh-cn.txt
└── README
cd)The command cd stands for Change Directory. It’s functionality is that it changes the current working directory into the one specified.
You can use it by typing in the following.
Note: replace <directory> with the specific directory you are trying to change into.
cd <directory>
Using cd with no arguments should bring you back to your home directory. (NOTE THIS WILL ONLY HAPPEN IF YOU ARE ON A UNIX BASED TERMINAL, ex: doesn’t work on Windows)
The current working directory is:
/home/lecture1
Example:
[user@john ~/lecture1]$
[user@john ~/lecture1]$ cd
[user@john ~]$
Result:
We had changed into the /home directory with no error from the /home/lecture1 directory and now if we do pwd we should get the output /home. We had gotten this result because we specific no directory. There had been no error message and our command executed, so there this is not an error. Extra: if we were to do this on a Windows terminal like Powershell, it wouldn’t do anything.
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What happens when you use cd on a directory path? It will just switch your working directory into the specified directory.
The current working directory is:
/home/lecture1
It’s file contents are (Note: messsages is a directory):
Hello.class Hello.java messages README
Example:
[user@john ~/lecture1]$ cd messages/
[user@john ~/lecture1/messages]$
Result:
With this command we have now switched into the /messages directory with no error. This had happend because we specificed to switch into the /messages directory. This isn’t an error because there is no error message and the command executed. Now if we run pwd, we can see our working directory is now, /home/lecture1/messages.
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What happens if you use cd on something that isn’t a directory? Simple, it will just prompt an error because the file specified isn’t a directory with file contents.
Our current working directory is:
/home/lecture1
It’s file contents are (Note: messsages is a directory):
Hello.class Hello.java messages README
Example:
[user@john ~/lecture1]$ cd README
bash: cd: README: Not a directory
[user@john ~/lecture1/]$
Result:
We had failed to switch our directory into README and had prompted and error because it is a file and not a directory. So the current working directory that we are in is still /home/lecture1 because our cd command was cancelled.
ls)The command ls stands for List. It’s functionality is that it lists all of the files in the current working directory that the user is in.
The following is how you use it. (The argument is optional)
ls <directory>
Using ls with no arguments should list out all of the non-hidden files in your current working directory.
Our current directory is:
/home/lecture1
It’s file contents are (Note: messsages is a directory):
Hello.class Hello.java messages README
Example:
[user@john ~/lecture1]$ ls
Hello.class Hello.java messages README
Result:
We had gotten a list of all of the items in the current working directory because we didn’t specify any arguments with no error. THe files listed out are all of the files in /home/lecture1 because that is the current working directory that we are in. This is not an error as we had no error message and the command executed.
#
What happens when you use ls on a directory path? It will list out all of the non-hidden files in that specific directory.
Our current working directory is:
/home/lecture1
It’s file contents are (Note: messsages is a directory):
Hello.class Hello.java messages README
Example:
[user@john ~/lecture1]$ ls
en-us.txt en-mx.txt zh-cn.txt
Result:
Because we had specified a directory we had listed all of the files in that directory. In this case we had specified the directory messages so we get a list of it’s file contents. There is no error because there is no error message and it had executed properly. We hadn’t switched into any other directory with this command so we are still in /home/lecture1.
#
What happens if you use ls on something that isn’t a directory? It will just list that specific file.
Our current directory is:
/home/lecture1
It’s file contents are (Note: messsages is a directory):
Hello.class Hello.java messages README
Example:
[user@john ~/lecture1]$ ls Hello.java
Hello.java
Result:
Because we specified a specific file not a directory, it had printed all of the contents, which is just a singular file, so the output was just our input which is Hello.java. This is not an error as there is no error message and the command had executed as expected. We hadn’t switched into any other directory with this command so we are still in /home/lecture1.
cat)The command cat is short for Concatenate. This command can take in several files as arguments (no directories) and prints out it’s contents into the terminal.
You can use it by typing the following: (can take in multiple args, arg is optional)
cat <file_name>
#
Using cat with no arguments will result in the terminal asking for user input, this user input will be echo’d back to the user. In order to exit this mode use the key-combination Ctrl+C.
Our current directory is:
/home/lecture1
Example: note: the blank space is the terminal asking for user input
[user@john ~/lecture1]$ cat
Example when the user gives input.
[user@john ~/lecture1]$ cat
Hello
Hello
woah
woah
^C
[user@john ~/lecture1]$
Result: Because we had used the command with no arguments, it had resulted in toggling a mode where it awaits the user to type something, then it echos it back to them. This is not an error as it is how the command is meant to function, there is also no error message, and no exception.
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What happens when you use cat on a directory path? It will prompt an error.
Our current directory is:
/home/lecture1
Example:
[user@john ~/lecture1]$ cat messages/
cat: messages/: Is a directory
Result:
Because we had used the command on the directory messages/ we had received an error. We had received this because the cat command can only take in files as arguments not directories.
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What happens if you use cat on something that isn’t a directory (using it on a file)? It will print out all of the text contents of that file onto the terminal.
Our current directory is:
/home/lecture1
Example:
[user@john ~/lecture1]$ cat Hello.java
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
public class Hello {
public static void main(String[] args) throws IOException {
String content = Files.readString(Path.of(args[0]), StandardCharsets.UTF_8);
System.out.println(content);
}
}
[user@john ~/lecture1]$
Result:
Because we had used the cat command on a specific file, in this case, Hello.java, it had printed out all of the contents inside that java file. This is not an error this is how the cat command was meant to work, there is no error message, and the command executed as expected.