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updating commands
puttaap Nov 5, 2021
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updating sort command flags
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updating sort command flags
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updating sort command flags
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updating emojis for important points
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Merge pull request #344 from theputta/dev_putta
sspencerwire Nov 8, 2021
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Suggested changes to chapter 3 - commands
sspencerwire Nov 8, 2021
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69 changes: 48 additions & 21 deletions docs/books/admin_guide/03-commands.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,8 @@
---
title: Linux Commands
author: Antoine Le Morvan
contributors: Steven Spencer, Aditya Putta
update: 11-08-2021
---

# Commands for Linux Users
Expand Down Expand Up @@ -178,13 +181,13 @@ This set of manuals is divided into 8 sections, grouping information by topic, t
8. System administration tools and daemons.
Information about each section can be accessed by typing `man x intro`, where `x` is the section number.

The command :
The command:

```bash
man passwd
```

will tell the administrator about the passwd command, its options, etc. While a :
will tell the administrator about the passwd command, its options, etc. While a:

```bash
$ man 5 passwd
Expand Down Expand Up @@ -215,7 +218,7 @@ Examples:
[root]# shutdown -r +5
```

Options :
Options:

| Options | Remarks |
| ------- | -------------------------------- |
Expand Down Expand Up @@ -270,7 +273,7 @@ The `clear` command clears the contents of the terminal screen. In fact, to be m

In a terminal, the display will be permanently hidden, whereas in a graphical interface, a scrollbar will allow you to go back in the history of the virtual terminal.

!!! tip
!!! Tip
<kbd>CTRL</kbd> + <kbd>L</kbd> will have the same effect as the `clear` command

### `echo` command
Expand All @@ -283,7 +286,7 @@ The `-n` option will not return to the line after displaying the text (which is

For various reasons, the script developer may need to use special sequences (starting with a `\` character). In this case, the `-e` option will be stipulated, allowing interpretation of the sequences.

Among the frequently used sequences, we can mention :
Among the frequently used sequences, we can mention:

| Sequence | Result |
| -------- | --------------------- |
Expand Down Expand Up @@ -396,7 +399,7 @@ The **relative path** references that same file by traversing the entire tree fr

In the above example, the "`..`" refers to the parent directory of the current directory.

A directory, even if it is empty, will necessarily contain at least **two references** :
A directory, even if it is empty, will necessarily contain at least **two references**:

* `.`: reference to itself.
* `..`: reference to the parent directory of the current directory.
Expand Down Expand Up @@ -458,7 +461,7 @@ $ ls /home
. .. rockstar
```

The main options of the `ls` command are :
The main options of the `ls` command are:

| Option | Information |
| ------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------ |
Expand Down Expand Up @@ -615,7 +618,7 @@ The `touch` command changes the timestamp of a file or creates an empty file if
touch [-t date] file
```

Example :
Example:

```bash
$ touch /home/rockstar/myfile
Expand Down Expand Up @@ -760,6 +763,7 @@ $ cp -r /home/rockstar /tmp
| `-f` | Do not ask for confirmation if overwriting the destination file. |
| `-p` | Keeps the owner, permissions and timestamp of the copied file. |
| `-r` | Copies a directory with its files and subdirectories. |
| `-s` | Creates a symbolik links rather than copying |

```bash
cp file1 /repexist/file2
Expand Down Expand Up @@ -850,7 +854,7 @@ The `cat` command concatenates the contents of multiple files and displays the r
cat file1 [files]
```

Example 1 - Displaying the contents of a file to the standard output :
Example 1 - Displaying the contents of a file to the standard output:

```bash
$ cat /etc/passwd
Expand All @@ -862,13 +866,13 @@ Example 2 - Displaying the contents of multiple files to standard output:
$ cat /etc/passwd /etc/group
```

Example 3 - Displaying the contents of several files in the file `usersAndGroups.txt` :
Example 3 - Displaying the contents of several files in the file `usersAndGroups.txt`:

```bash
$ cat /etc/passwd /etc/group > usersAndGroups.txt
```

Example 4 - Displaying the line numbering :
Example 4 - Displaying the line numbering:

```bash
$ cat -n /etc/profile
Expand Down Expand Up @@ -951,7 +955,7 @@ The `sort` command sorts the lines of a file.
It allows you to order the result of a command or the content of a file in a given order, numerically, alphabetically, by size (KB, MB, GB) or in reverse order.

```bash
sort [-kx] [-n] [-o file] [-ty] file
sort [-kx] [-n] [-u] [-o file] [-ty] file
```

Example:
Expand All @@ -969,6 +973,7 @@ adm:x:3:4:adm:/var/adm/:/sbin/nologin
| `-o file ` | Saves the sort to the specified file |
| `-ty` | Specifies the field separator character `y`. |
| `-r` | Reverse the order of the result |
| `- u` | unique |

The `sort` command sorts the file only on the screen. The file is not modified by the sorting. To save the sort, use the `-o` option or an output redirection `>`.

Expand Down Expand Up @@ -998,7 +1003,7 @@ $ sort -R /etc/passwd

A system administrator is quickly confronted with the processing of IP addresses from the logs of his services such as SMTP, VSFTP or Apache. These addresses are typically extracted with the `cut` command.

Here is an example with the file `dns-client.txt` :
Here is an example with the file `dns-client.txt`:

```
192.168.1.10
Expand All @@ -1017,11 +1022,32 @@ $ sort -nr dns-client.txt
5.1.150.146
```

* Sorting file by removing duplicates

The `sort` command knows how to remove the duplicates from the file output using `-u` as option.

Here is an example with the file `colours.txt`:

```
Red
Green
Blue
Red
Pink
```
```
$ sort -u colours.txt
Blue
Green
Pink
Red
```

* Sorting file by sizes

The `sort` command knows how to recognize file sizes, from commands like `ls` with the `-h` option.

Here is an example with the file `size.txt` :
Here is an example with the file `size.txt`:

```
1,7G
Expand Down Expand Up @@ -1094,12 +1120,13 @@ $ find /tmp -name *.txt -exec rm -f {} \;

The previous command searches for all files in the `/tmp` directory named `*.txt` and deletes them.


!!! Tip "Understand the `-exec` option"
In the example above, the `find` command will construct a string representing the command to be executed.

If the `find` command finds three files named `log1.txt`, `log2.txt`, and `log3.txt`, then the `find` command will construct the string by replacing in the string `rm -f {} \;` the braces with one of the results of the search, and do this as many times as there are results.

This will give us :
This will give us:

```
rm -f /tmp/log1.txt ; rm -f /tmp/log2.txt ; rm -f /tmp/log3.txt ;
Expand Down Expand Up @@ -1301,7 +1328,7 @@ $ ls -R / 2>> errors_file

### Examples of redirection

Redirection of 2 outputs to 2 files :
Redirection of 2 outputs to 2 files:

```bash
$ ls -R / >> ok_file 2>> nok_file
Expand All @@ -1313,7 +1340,7 @@ Redirection of the 2 outputs to a single file:
$ ls -R / >> log_file 2>&1
```

Redirection of *stderr* to a "bottomless pit" (`/dev/null`) :
Redirection of *stderr* to a "bottomless pit" (`/dev/null`):

```bash
$ ls -R / 2>> /dev/null
Expand Down Expand Up @@ -1390,13 +1417,13 @@ For example:
$ ll
```

will replace the command :
will replace the command:

```bash
$ ls -l
```

The `alias` command lists the aliases for the current session. Aliases are set by default on Linux distributions. Here, the aliases for a Rocky server :
The `alias` command lists the aliases for the current session. Aliases are set by default on Linux distributions. Here, the aliases for a Rocky server:

```bash
$ alias
Expand All @@ -1409,13 +1436,13 @@ alias which='alias | /usr/bin/which --tty-only --read-alias --show-dot --show-ti

The aliases are only defined temporarily, for the time of the user session.

For permanent use, they must be created in the :
For permanent use, they must be created in the:

* `.bashrc` file in the user's login directory;
* `/etc/profile.d/alias.sh` file for all users.

!!! Warning
Special care must be taken when using aliases which can be potentially dangerous! For example, an alias set up without the administrator's knowledge :
Special care must be taken when using aliases which can be potentially dangerous! For example, an alias set up without the administrator's knowledge:

```bash
alias cd='rm -Rf'
Expand Down