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chis
Mission Specialist
Mission Specialist
  • 1,070 Views

Chapter01s07

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I feel like this line could use some extra explanation, I saw nothing related to this in the preceding pages.

ssh ${USR}@${SRV} "grep -v '^$' /etc/selinux/config|grep -v '^#'" >> ${OUT}-${SRV}

Specifically

'^$' 

- I assume this is the expression that removes whitespaces, but how or why and where is this mentioned in the preceding guide.

If it is expected to do our own research, that is fine, but it does not suggest you should do this.

Please advise.

Thanks.

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chis
Mission Specialist
Mission Specialist
  • 1,009 Views

Hi Chetan,

Thank you for providing further information.

I had conflicting understanding of the $ flag.

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6 Replies
Chetan_Tiwary_
Community Manager
Community Manager
  • 1,063 Views

Hi @chis , thanks for the feedback, Let me check it and get back to you.

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Chetan_Tiwary_
Community Manager
Community Manager
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ok @chis , I think you are talking about step 2.2 :

Chetan_Tiwary__0-1729526604864.png

The command grep -v '^$' /etc/selinux/config | grep -v '^#' searches the /etc/selinux/config file on the remote server for lines that are not empty and do not start with a # character (which usually indicates a comment). The -v option in both grep commands inverts the match, meaning it finds lines that don't match the specified patterns. This ensures that only non-empty, non-comment lines are extracted from the configuration file.

I will file an improvement request to ensure there is an explanation for this script in the course steps.

Thank you for this feedback!

 

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chis
Mission Specialist
Mission Specialist
  • 1,010 Views

Hi Chetan,

Thank you for providing further information.

I had conflicting understanding of the $ flag.

Chetan_Tiwary_
Community Manager
Community Manager
  • 997 Views

pleasure @chis !

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samatar_farah
Mission Specialist
Mission Specialist
  • 469 Views

Yeah, you and me both! The previous videos explained grep and the "for i in..." stuff well. Hopefully, Ricardo can add a couple extra slides in furthering more detail in his demo based systems regarding this.

RunReon
Mission Specialist
Mission Specialist
  • 431 Views

Just sharing my learning experience. When I was desperately looking for an explanation and nobody was yet available to help, I asked chatgpt.com and below is its answer.

I typed:
Breakdown the following,
ssh ${USR}@${SRV} "grep -v '^$' /etc/selinux/config|grep -v '^#'" >> ${OUT}-${SRV}

Responsed from chatgpt.com:

The command you've provided is a one-liner that performs a sequence of operations remotely using SSH and manipulates the output using `grep`. Here's a breakdown of each part of the command:

```bash
ssh ${USR}@${SRV} "grep -v '^$' /etc/selinux/config | grep -v '^#'" >> ${OUT}-${SRV}
```

### 1. **`ssh ${USR}@${SRV}`**
- **`ssh`**: This is the **Secure Shell (SSH)** command used to log into a remote machine securely over a network.
- **`${USR}`**: This is a variable that represents the username used to log into the remote server. It could be something like `root`, `user`, or any valid username.
- **`${SRV}`**: This is a variable that represents the remote server's hostname or IP address. For example, it might be `example.com` or `192.168.1.100`.

So, the command initiates an SSH session to the remote machine `${SRV}` as the user `${USR}`.

### 2. **`"grep -v '^$' /etc/selinux/config | grep -v '^#'"`**
This is the command that will be executed on the remote server, enclosed in double quotes. It is a pipeline of two `grep` commands:

- **`grep -v '^$' /etc/selinux/config`**:
- **`grep`**: This is the command used for searching through files based on patterns.
- **`-v`**: This option tells `grep` to invert the match, i.e., exclude lines that match the given pattern.
- **`'^$'`**: This is the regular expression that matches empty lines. The `^` asserts the start of a line, and `$` asserts the end of the line, so `^$` matches lines with no characters (empty lines).
- **`/etc/selinux/config`**: This is the file being searched. In this case, it's the SELinux configuration file.

So, this part of the command will **exclude empty lines** from the `/etc/selinux/config` file.

- **`|` (Pipe)**: The pipe `|` takes the output of the first `grep` command and passes it as input to the second `grep` command.

- **`grep -v '^#'`**:
- **`grep -v`**: Again, this tells `grep` to exclude lines that match the pattern.
- **`'^#'`**: This regular expression matches lines that start with a `#` character. In the context of the SELinux config file, lines starting with `#` are usually comments.

So, this second `grep` will **exclude comment lines** (lines starting with `#`).

In summary, the pipeline of these two `grep` commands will:
- **Exclude empty lines** (lines with no content).
- **Exclude comment lines** (lines starting with `#`).
- This leaves only the active, non-commented lines of the `/etc/selinux/config` file.

### 3. **`>> ${OUT}-${SRV}`**
- **`>>`**: This is the **append output operator** in the shell. It redirects the output of the preceding command into a file and appends it to the file if it already exists. If the file doesn't exist, it will be created.
- **`${OUT}`**: This is a variable, presumably set earlier in the script, that represents the base name of the output file.
- **`${SRV}`**: This variable represents the name of the remote server, so the final output file will be named something like `output-servername` (where `output` is the value of `${OUT}` and `servername` is the value of `${SRV}`).

This part of the command appends the filtered output from the remote `grep` commands into a file named `${OUT}-${SRV}`.

### Putting it all together:
- **SSH**: Logs into the remote server `${SRV}` as the user `${USR}`.
- **Command execution**: Once logged in, it runs a `grep` pipeline to:
- Exclude empty lines (`grep -v '^$'`).
- Exclude comment lines (`grep -v '^#'`).
- This processes the `/etc/selinux/config` file to get only the relevant configuration lines.
- **Output redirection**: The resulting output from the `grep` commands is appended to a local file named `${OUT}-${SRV}`.

 

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