add comments on pipes (#1502)

Author: pdjota

common-content/en/module/tools/individual-shell-tools/index.md

@@ -15,9 +15,34 @@ Through this course, we are going to write lots of programs.
 
 But a lot of programs have already been written by other people. It's often quicker and easier to use an existing program than to write a new one.
 
-We are going to learn about some commonly used programs. We will learn some common patterns that make them easier to learn them and to combine them.
+Many programs that were written for Unix (the predecessor of Linux) are still used today. Why are these programs still useful today? In part because of the [Unix philosophy](https://en.wikipedia.org/wiki/Unix_philosophy), the way these programs were written: simple, modular and flexible.
 
-We run these programs from a terminal. We can either call the programs directly from a terminal, or put instructions to call the programs in a script and run the script.
+- **Simple**: Each tool solves a specific problem, making it easier to understand and learn.
+- **Modular**: We can reuse tools in different ways by combining them.
+- **Flexible**: Pipelines let us build solutions quickly without coding from scratch.
+
+We are going to learn about some commonly used programs. We will learn common patterns that make them easier to learn and combine.
+
+We run these programs from a terminal. We can either call the programs directly from a terminal, or write the instructions to call the programs in a script and run the script.
+
+### Key Concepts
+
+**Pipelines**: We can connect programs together so the output of one becomes the input of another using the `|` (pipe) symbol.
+
+For example, to find lines containing "error" in all .log files and show how many unique files have them:
+
+```console
+% grep -l "error" *.log | wc -l
+3
+```
+
+Here, `grep -l "error" *.log` finds all .log files containing "error" and outputs just the filenames, then `wc -l` counts the lines (filenames) to give us the total count.
+
+**PATH**: The system searches for programs in directories listed in the PATH environment variable. When you type a command, the shell looks for the program in these directories.
+
+**Important**: The current directory (`.`) is not in PATH by default for security reasons. To run a program in the current directory, you must use `./program-name` instead of just `program-name`.
+
+**Finding programs**: Use `which <program-name>` to see where a program is located.
 
 ### Conventions
 
@@ -31,24 +56,24 @@ file1   file2
 Here we are saying: At a terminal prompt (signified by the `%`), run the command `ls`.\
 Below the command is the output (`file1    file2`) you should expect to see after you run the command. Sometimes this is the exact output you should expect. Sometimes _example_ outputs are given: these could vary according to your filesystem, username, or other variables.
 
-To call a program, we type its name and press enter.  Open a terminal and run:
+To call a program, we type its name and press enter. Open a terminal and run:
 
 ```console
 % ls
 file1   file2
 ```
 
-Most of the programs which take a file as an input take it as an argument to the command.\
-We list arguments after the command name, separated with a space, e.g. we can pass `/tmp` as an argument to `ls`:
+Most programs that take a file as input accept it as an argument to the command.\
+We list arguments after the command name, separated with a space. For example, we can pass `/tmp` as an argument to `ls`:
 
 ```console
 % ls /tmp
 some-temporary-file some-other-temporary file
 ```
 
-#### Flags and arguments
+#### Flags and Arguments
 
-Some programs take extra arguments. These may be optional or required. Extra arguments generally start with a `-` or `--` then their name.  If these extra arguments take a value, expect the value to come _after_ the name.
+Some programs take extra arguments. These may be optional or required. Extra arguments generally start with a `-` or `--` followed by their name. If these extra arguments take a value, expect the value to come _after_ the name.
 
 ```console
 % grep -r -B 1 "hello" .
@@ -59,10 +84,10 @@ Some programs take extra arguments. These may be optional or required. Extra arg
 ./errors/index.md:const result = console.log("hello world");
 ```
 
-Here we passed the optional argument `-r` and the optional argument `-B`. `-r` takes no value. We gave `-B` the value 1. 
+Here we passed the optional argument `-r` and the optional argument `-B`. `-r` takes no value. We gave `-B` the value 1.
 
-We call arguments that start with a `-` or `--` flags. We could give flags in any order - `grep -B 1 -r "hello" .` works the same as `grep -r -B 1 "hello" .`.
+We call arguments that start with a `-` or `--` **flags**. We could give flags in any order - `grep -B 1 -r "hello" .` works the same as `grep -r -B 1 "hello" .`.
 
-Flags that start with a `-` not a `--` can also be squashed together. `grep -rc "Hello" .` works the same as `grep -r -c "Hello" .`. If you are looking up a flag you don't know, like `-rc`, remember it may actaully be two flags (`-r` and `-c`). You can also specify a value for the _last_ flag in a squashed together list: `grep -rcB 1 "hello" .`.
+Flags that start with a `-` (not `--`) can also be combined together. `grep -rc "Hello" .` works the same as `grep -r -c "Hello" .`. If you are looking up a flag you don't know, like `-rc`, remember it may actually be two flags (`-r` and `-c`). You can also specify a value for the _last_ flag in a combined list: `grep -rcB 1 "hello" .`.
 
-`"hello"` and `.` are called positional arguments because they don't have a named flag before them. The program decides how to interpret them based on their position (order) in the command line, rather than a named flag coming before them. Their order matters.
+`"hello"` and `.` are called **positional arguments** because they don't have a named flag before them. The program decides how to interpret them based on their position (order) in the command line, rather than a named flag coming before them. Their order matters.