Mystery of shell and exec form in bash

Keep calm and read my conversation with chatgpt 4

what happens behind the scenes when a inetrnal and external command is executed. I want to know how it moves from ram to harddisk and also total number of processes spawned?

Sure, let's break this down step by step:

1. Command Input:

Whenever you type in a command in the terminal and press Enter:

The shell first parses the command to understand its structure (like which parts are commands, arguments, redirections, etc.).
It then checks for any aliases that might replace the typed command.
The shell searches for the command in its list of built-ins.

2. Internal (Built-in) Command Execution:

If the command is found to be an internal command:

The shell directly executes the command within its own process. No new processes are spawned for this.
The command doesn't move between RAM and hard disk specifically for execution, as the shell is already loaded into RAM and the built-in command's logic is part of the shell's code.
Once the command finishes execution, control is returned to the shell for the next command or operation.

3. External Command Execution:

If the command isn't an internal command:

The shell uses the system's PATH variable to search for the command in directories specified by PATH.
When it finds the appropriate executable, the shell spawns a new process to run this external command. This is done using the fork() system call, which creates a child process as a duplicate of the shell process.
The child process, which is still a copy of the shell, then uses the exec() system call to replace its image with the image of the external command.
Now, about the "RAM to hard disk" part: the executable binary of the external command resides on the hard disk. When you execute it, the operating system loads the binary into RAM for execution. If this binary was previously executed and still resides in the RAM (because of OS caching mechanisms), the OS might skip the hard disk read. But if it's the first time or if the binary was purged from RAM, it needs to be read from the disk.
Once the external command finishes execution, the child process terminates. The parent process (the original shell) remains and waits for more commands.

Number of Processes Spawned:

For internal commands: No new processes are spawned. The command runs in the context of the current shell process.
For external commands: Typically, one new process is spawned for the command. However, if the command itself spawns other processes, then more than one process will be created. For example, commands that start background services might spawn multiple processes.

Note: For complex command lines with pipes, background tasks (&), and command substitutions ($(...)), more processes can be spawned. For instance, command1 | command2 will spawn separate processes for command1 and command2.

This sequence of events ensures that commands are executed efficiently, with built-ins offering fast execution without the overhead of process creation, and external commands being flexible and diverse, able to be developed and installed independently of the shell.

In external command flow what happens to parent shell process until child process gets executed. Does it waits or executes other commands . I also wanted to know when a command spawns more than single process i.e pipes, command substitutions etc?

ChatGPT

Great questions! Let's break this down.

1. Parent Shell Process and Child Process Interaction:

When the shell spawns a child process to run an external command:

By default, the parent shell process (the one from which you issued the command) waits for the child process to complete its execution. During this time, the shell does not execute other commands; it's in a waiting state.
Once the child process completes its execution and exits, the control returns to the parent shell process, and you'll see your shell prompt again, ready to accept new commands.

2. Running Processes in the Background:

However, you can control this behavior:

If you append an & to your command, you instruct the shell to run the command in the background. In this case, the shell does not wait for the child process to complete. It immediately returns control to you, and you can continue to execute other commands. The backgrounded command runs concurrently.

Example:
```
  bashCopy codesleep 10 &
```
This will run the sleep command in the background, and you'll get the shell prompt back immediately.

3. Commands with Pipes, Command Substitutions, etc.:

When you use constructs like pipes (|) or command substitutions ($(...)):

Pipes: Each command in the pipeline runs in its own process. So, for a command like command1 | command2, two processes are spawned: one for command1 and another for command2. The shell waits for all processes in the pipeline to complete before returning control to you. The output of command1 is passed as input to command2 through an inter-process communication mechanism.
Command Substitutions: When you use $(...), the command inside the parentheses is executed in a subshell (a new process). The output of that command replaces the $(...) construct in the original command. Once the command inside $(...) completes, the parent command continues its execution.

Example:
```
  bashCopy codeecho "Today is $(date)"
```
Here, date runs in a subshell, and its output replaces $(date), so you might get an output like "Today is Sun Sep 26 12:00:00 UTC 2022".
Other constructs: Other shell constructs, like command lists separated by semicolons (;), or boolean operators (&& and ||), do not spawn additional processes unless the commands themselves are external commands. They're primarily used for controlling the flow of command execution in the shell.

Summary:

By default, the parent shell process waits for the child process to complete. You won't be able to execute other commands in that specific shell until the child process exits, unless you've backgrounded the process or are using some form of multitasking mechanism (like using multiple terminal tabs or screen sessions).
Constructs like pipes and command substitutions do spawn multiple processes, and the parent shell coordinates the flow of data and control among them.

Nice I want a bash script(not too small), which has bunch of commands. I want you to explain the flow i.e when a line is executed process spawns, internal and exteral commands, piipes,redirections and command substitutions etc

ChatGPT

Certainly! Let's create a bash script that utilizes a variety of constructs, and then we'll walk through its execution step-by-step.

Sample Bash Script:

bashCopy code#!/bin/bash

echo "Starting the script..."

# Define a variable
current_time=$(date)

# Print the variable
echo "Current time is: $current_time"

# Create a file and list its details using a pipeline
touch temp_file.txt
echo "Sample data" > temp_file.txt
ls -l temp_file.txt | grep "temp_file.txt"

# Command substitution with an external command
number_of_lines=$(wc -l < temp_file.txt)
echo "Number of lines in temp_file.txt: $number_of_lines"

# Redirections and pipes
cat temp_file.txt | sort > sorted_temp_file.txt

# Using an internal command
cd ..
current_directory=$(pwd)
echo "We've moved to directory: $current_directory"

echo "Ending the script..."

Step-by-Step Explanation:

#!/bin/bash: This is called a shebang. It's not executed like other lines. It simply tells the operating system that this script should be run using the bash shell.
echo "Starting the script...": This is an external command. The shell spawns a new process to run the echo command.
current_time=$(date): This uses command substitution. The date command runs in a subshell, and its output is captured and assigned to the current_time variable.
echo "Current time is: $current_time": External command (echo). The variable $current_time is expanded by the shell before the echo command is executed.
touch temp_file.txt: External command (touch). It creates an empty file named temp_file.txt.
echo "Sample data" > temp_file.txt: This is a combination of an external command (echo) and redirection (>). The output of echo is redirected to temp_file.txt.
ls -l temp_file.txt | grep "temp_file.txt": This is a pipeline with two external commands. ls lists the details of temp_file.txt and its output is piped as input to grep, which filters the output.
number_of_lines=$(wc -l < temp_file.txt): Command substitution with external command (wc) and input redirection (<). It calculates the number of lines in the file.
echo "Number of lines in temp_file.txt: $number_of_lines": External command (echo). The variable $number_of_lines is expanded by the shell before the echo command is executed.
cat temp_file.txt | sort > sorted_temp_file.txt: This combines an external command (cat), a pipe (|), another external command (sort), and output redirection (>). The content of temp_file.txt is displayed, sorted, and then saved to a new file.
cd ..: This is an internal command. No new process is spawned. It changes the current working directory of the script to the parent directory.
current_directory=$(pwd): Command substitution with an external command (pwd). It gets the current working directory and assigns it to a variable.
echo "We've moved to directory: $current_directory": External command (echo). The variable $current_directory is expanded by the shell before the echo command is executed.
echo "Ending the script...": External command (echo).

Each time an external command is executed, a new process is spawned. Constructs like command substitutions or pipes can lead to multiple processes being spawned in a single line. Internal commands, on the other hand, are executed within the shell's process itself.

I want more in detail set by step explanation. I know what the command is doing. I am intersted in if is a internal or external and how many child procees it created(each instructions). what is parent process and what is child process etc

ChatGPT

Certainly! Let's go step by step through the script you provided, focusing on the details you've requested.

Step-by-Step Analysis:

1. #!/bin/bash

This is a shebang. It does not spawn a process. It indicates that the script should be executed using the bash shell.

2. echo "Starting the script..."

echo is an external command.
Parent process (the shell executing the script) spawns one child process for echo.
Once the echo command completes, the child process terminates, and control returns to the parent.

3. current_time=$(date)

date is an external command.
The command substitution ($(...)) causes the shell to spawn a subshell (child process) to execute date.
Once date finishes, the subshell terminates, and the output is captured into the variable current_time.

4. echo "Current time is: $current_time"

echo is external.
The shell spawns a child process for echo. Variable expansion occurs in the parent shell before echo receives it as an argument.

5. touch temp_file.txt

touch is an external command.
The shell spawns one child process to execute touch.

6. echo "Sample data" > temp_file.txt

echo is external.
The shell spawns one child process for echo.
The redirection (>) is handled by the shell itself (in the parent process), directing the output of echo to the file.

7. ls -l temp_file.txt | grep "temp_file.txt"

Both ls and grep are external commands.
The shell spawns a child process for ls.
Simultaneously, another child process is spawned for grep.
A pipe (|) connects the output of ls to the input of grep. The pipe is set up by the parent shell process.
Once both ls and grep complete, their processes terminate.

8. number_of_lines=$(wc -l < temp_file.txt)

wc is an external command.
The shell spawns a subshell for the command substitution.
Within the subshell, another child process is spawned to execute wc.
The input redirection (<) is handled by the shell, directing the content of temp_file.txt to wc.

9. echo "Number of lines in temp_file.txt: $number_of_lines"

echo is external.
The shell spawns a child process for echo.

10. cat temp_file.txt | sort > sorted_temp_file.txt

Both cat and sort are external commands.
The shell spawns a child process for cat.
Another child process is spawned for sort.
The output of cat is piped to sort, and the sorted output is redirected to the file.

11. cd ..

cd is an internal command.
The shell does not spawn any new process. It changes its own current working directory.

12. current_directory=$(pwd)

pwd is an external command.
A subshell is spawned for the command substitution.
Within that subshell, a child process is spawned for pwd.

13. echo "We've moved to directory: $current_directory"

echo is external.
The shell spawns a child process for echo.

14. echo "Ending the script..."

echo is external.
The shell spawns a child process for echo.

Summary:

Internal commands (cd in this script) do not spawn new processes.
External commands always cause the parent shell process to spawn a new child process.
Command substitutions ($(...)) cause a subshell to be spawned, and any external commands inside will also get their own child processes.
Pipelines (|) cause multiple processes to be spawned – one for each command in the pipeline.

The "parent process" in this context is the shell process that's interpreting and running the script. Every time an external command is executed, the parent spawns a new "child process" for that command.

External commands always cause the parent shell process to spawn a new child process Command substitutions ($(...)) cause a subshell to be spawned, and any external commands inside will also get their own child processes. Does child process and subshell are different?

ChatGPT

Yes, the terms "child process" and "subshell" have distinct meanings, though they can sometimes be used in contexts where they seem similar. Let's clarify:

Child Process:
- Whenever the shell executes an external command, it creates a new process, separate from itself, to run that command. This new process is a "child process" of the shell.
- A child process can run both shell scripts and non-shell programs.
- A child process has its own process ID and runs independently of the parent process, though the parent typically waits for the child to finish before continuing (unless the child is run in the background).
- The shell itself, when started, is a child process of whatever initiated it (e.g., another shell, a terminal emulator, an init system, etc.).
Subshell:
- A subshell is essentially a new instance of the shell. It's a child process, but it's a special kind of child process because it's a replica of the parent shell.
- Command substitutions ($(...) and ...) and shell operations that require a separate environment (like parts of scripts executed in the background or commands executed in parentheses) run in a subshell.
- Variables set in a subshell are not visible in the parent shell, because the subshell is a separate environment. For instance, in the command a=$(b=3; echo $b), the variable b is set in the subshell, but its value wouldn't be accessible outside the command substitution.
- A subshell inherits variables and the environment from its parent, but changes made in the subshell don't affect the parent.

So, while all subshells are child processes (of the parent shell), not all child processes are subshells. The term "subshell" is specifically used when referring to a new instance of the shell itself, created from a parent shell.

Now You got enough information get into the actual topic

Now I got clear picture. Now explain me exec form and shell form in docker. How it is related to this entire linux/bash concept explain me in detail

ChatGPT

Certainly! The concepts of exec form and shell form in Docker tie directly into the aforementioned details about shell processes and child processes.

1. Shell vs. Exec Form in Docker:

In a Dockerfile, when you're specifying commands such as CMD and ENTRYPOINT, you can define them in either shell form or exec form.

Shell Form:
```
   command param1 param2
```
or
```
  ENTRYPOINT command param1 param2
```
In this form, the command is run in a shell - /bin/sh -c. So, the given command spawns a shell (sh) as its parent, which then executes the command.
Exec Form:
```
  CMD ["executable", "param1", "param2"]
```
or
```
  ENTRYPOINT ["executable", "param1", "param2"]
```
In this form, the command is executed without a shell. The command becomes the PID 1 of the container and doesn't spawn any additional processes.

2. How It Relates to the Linux/Bash Concept:

Shell Form:
- Spawns a shell as an intermediate layer.
- This is similar to executing an external command in a bash script, where the shell script's process would spawn a child process (the shell) to execute the command.
- Signals: If you send a signal to the container, it'll be received by the shell first, not the actual command. This can sometimes cause unexpected behavior, especially with the graceful termination of processes.
Exec Form:
- Doesn't involve a shell; the command runs directly.
- This is similar to an internal command in a shell script, which doesn't spawn a new process.
- Signals: The command will directly receive any signals sent to the container. This is often preferable, especially when you want your containerized application to handle signals like SIGTERM gracefully.

3. Practical Implications:

Efficiency: The exec form is generally more efficient because it doesn't need to spawn an extra shell process.
Signal Handling: If you're running applications that need to handle UNIX signals, the exec form is typically better, as the process will receive signals directly.
Scripting and Expansion: If you need environment variable expansion or want to run a series of commands, the shell form provides those capabilities since it runs within a shell.

4. Conclusion:

Understanding the difference between shell and exec forms in Docker is crucial for crafting optimized and effective container images. It directly ties back to the concepts of processes in Linux and how shells handle command execution. By understanding the underpinnings, you can make more informed decisions about how to structure your Docker commands, ensuring that your containers run efficiently and handle signals as expected.

In my container entrypoint i want to run a bash script then which is preferred explain with example

ChatGPT

If you want to run a bash script as the entrypoint of your container, you have a couple of options. The best approach typically depends on the specifics of your needs. Let's look at both approaches:

1. Shell Form:

The shell form of ENTRYPOINT is more flexible and allows for environment variable expansion and chaining commands using shell syntax. It's particularly useful if the script needs to be dynamic based on container environment variables.

Dockerfile example:

 FROM ubuntu:latest

COPY ./my_script.sh /my_script.sh
RUN chmod +x /my_script.sh

ENTRYPOINT /my_script.sh

When the container is started, it will invoke /bin/sh -c /my_script.sh under the hood.

2. Exec Form:

The exec form does not involve a shell layer and runs the specified command (in this case, the script) directly. This makes it slightly more efficient, and signal handling is more direct. However, you won't have the shell's features like environment variable expansion within the Dockerfile's ENTRYPOINT directive itself.

Dockerfile example:

FROM ubuntu:latest

COPY ./my_script.sh /my_script.sh
RUN chmod +x /my_script.sh

ENTRYPOINT ["/my_script.sh"]

When the container is started, it will run /my_script.sh directly without spawning a shell.

Which is Preferred?

If your bash script relies on shell-specific features, like environment variable expansion within the Dockerfile's ENTRYPOINT directive or you have chained commands, then the shell form is more suitable.
If you need the script to handle UNIX signals directly (for example, if your script starts a process that should receive a SIGTERM when the container is stopped), the exec form is more appropriate.
For simplicity and better signal handling, the exec form is generally preferred. Just ensure your bash script has the appropriate shebang (#!/bin/bash or #!/bin/sh), so it's executed with the correct interpreter.

Considering the above points, if you're just running a straightforward bash script as the main process in your container, the exec form is typically the preferred method.

If i use shell form then pid1 proccess inside container will be shell instead of my script right? but in exec from pid 1 will be myscript. am ia correct?

ChatGPT

Yes, you're absolutely correct! Let's break it down:

1. Shell Form:

When you use the shell form in a Dockerfile:

ENTRYPOINT /my_script.sh

The Docker container starts the shell (typically /bin/sh) as the main process with PID 1. This shell then executes your script. So, inside the container:

PID 1 will be /bin/sh
Your script (/my_script.sh) will be run by this shell and will have a different PID (greater than 1).

2. Exec Form:

When you use the exec form:

ENTRYPOINT ["/my_script.sh"]

Docker runs the specified command (in this case, your script) directly without involving an intermediate shell. Inside the container:

PID 1 will be /my_script.sh

Your understanding is spot-on. The implications of this difference are primarily in signal handling and the overall process hierarchy inside the container. The exec form ensures that signals sent to the container are received directly by your script, which can be crucial for proper application shutdown and resource cleanup.

After pid 1 is completed then container will be killed right?

ChatGPT

Yes, that's correct. In a Docker container, the lifecycle of the container is tied to the lifecycle of the process with PID 1. Once the PID 1 process exits, the container is considered "stopped" or "completed," and it will no longer be running.

A few key points to remember:

PID 1 Importance: The process with PID 1 inside a container is treated specially. It's the first process that gets started, and it's expected to manage all other processes in the container. This is similar to how the init process works on a traditional Linux system.
Signal Handling: The process with PID 1 receives any signals sent to the container. This is why proper signal handling is important, especially if you want your application to shut down gracefully when the container is stopped.
Container Lifecycle: When the PID 1 process exits for any reason (either naturally because it completed its task or because it crashed), the container will also exit. If you run docker ps, you'll no longer see the container in the list of running containers.

This behavior highlights the importance of ensuring that the main application or service inside a container runs as PID 1, especially if you want to have more direct control over its lifecycle and signal handling.

Explain me with exmple how to write same command in exec and shell form like echo secuting script, running a background process etc

ChatGPT

Sure, I'll illustrate the distinction between the exec and shell forms in a Dockerfile using a few examples:

Example 1: Basic Command

Objective: We want to echo "Executing script..."

Shell Form:

DockerfileCopy codeCMD echo "Executing script..."

This gets translated to /bin/sh -c "echo 'Executing script...'" inside the container.

Exec Form:

DockerfileCopy codeCMD ["echo", "Executing script..."]

This directly runs the echo command without an intermediate shell.

Example 2: Running a Background Process

Objective: We want to start a background process.

Shell Form:

DockerfileCopy codeCMD service nginx start && tail -f /var/log/nginx/access.log

This gets translated to /bin/sh -c "service nginx start && tail -f /var/log/nginx/access.log" inside the container.

Exec Form:

DockerfileCopy codeCMD ["sh", "-c", "service nginx start && tail -f /var/log/nginx/access.log"]

Here, we explicitly invoke the sh shell and pass the entire command as a single string argument to sh -c.

Example 3: Script Execution

Objective: We want to execute a script located at /app/start.sh.

Shell Form:

DockerfileCopy codeCMD /app/start.sh

This becomes /bin/sh -c "/app/start.sh" inside the container.

Exec Form:

DockerfileCopy codeCMD ["/app/start.sh"]

This directly executes the script /app/start.sh without a wrapping shell. Ensure the script has a proper shebang (like #!/bin/bash) at the beginning.

Note:

While the distinction between these forms might seem subtle, remember that it affects aspects like environment variable expansion and signal handling in the container:

Environment Variable Expansion: Only the shell form will do inline environment variable expansion in the Dockerfile. For example, CMD echo $HOME will work in shell form but not in exec form without an explicit shell.
Signal Handling: If you need your process to directly handle signals (e.g., SIGTERM), the exec form is preferable because it avoids an intermediate shell process.

By understanding the nuances of these two forms, you can tailor your Docker containers to your specific needs.

In example 1: exec form .. echo command without an intermediate shell. Obviously echo command execution requires a shell right ?? how can we execute echo command without a shell?

ChatGPT

You're right in your observation, and I appreciate the keen eye. The echo command can be a bit of a special case because echo exists both as a shell built-in command and as an external binary in many systems. However, the distinction can sometimes lead to confusion.

In many Linux distributions, there's an /bin/echo (or sometimes /usr/bin/echo) binary. This is an external command, and it doesn't require a shell for execution. When you use the exec form in Docker with the echo command:

CMD ["echo", "Executing script..."]

Docker will attempt to execute the echo binary directly (i.e., /bin/echo), bypassing any shell. If the binary exists, it will work; if not, it will fail.

However, when you're working in a shell session and you type echo, you're typically using the shell's built-in version of echo, not the external binary. Built-in commands are executed directly by the shell and do not spawn a separate process. You can check which echo you're using with the type command:

$ type echo
echo is a shell builtin

$ type echo
echo is /bin/echo

To your point, if a system only had echo as a shell built-in and lacked the external /bin/echo binary, then the exec form in Docker would indeed fail. However, many Linux base images used in Docker containers include the /bin/echo binary, making the exec form valid.

Always ensure that the command you're using in the exec form exists as an external binary if you expect it to run without a shell.

In example 2 why are we explicitly invoking shell?

ChatGPT

In Example 2, the command we want to run contains shell-specific features, namely:

Chaining Commands with &&: The use of && to chain commands is a feature provided by the shell. It means "run the command on the right only if the command on the left succeeds."
Background Processes: While not explicitly shown in that example, if you were to use & to run a process in the background, that too is a shell feature.

Because these features are specific to the shell and are not innate properties of the commands themselves, we need a shell to interpret and execute them. Thus, in the exec form, which doesn't use a shell by default, we need to explicitly invoke one using sh -c.

Here's that portion of Example 2 again for reference:

Exec Form:

CMD ["sh", "-c", "service nginx start && tail -f /var/log/nginx/access.log"]

By using ["sh", "-c", "command"], we're saying:

Run the sh binary (which is the shell).
Use the -c option of sh to pass in a command as a string.
The command string to run is "service nginx start && tail -f /var/log/nginx/access.log".

This ensures that the provided command string is interpreted and executed by the shell, allowing us to use shell-specific features within the exec form of a Docker instruction.