When working with Docker containers, developers often encounter a puzzling behavior where environment variables (particularly PATH) show different values between these two scenarios:

# Method 1: Direct command execution
docker exec -it my_container bash -c "echo $PATH"

# Method 2: Interactive shell
docker exec -it my_container bash
root@container_id:/# echo $PATH

The root cause lies in how bash loads configuration files:

• Interactive login shells read: /etc/profile → ~/.bash_profile → ~/.bash_login → ~/.profile
• Interactive non-login shells read: ~/.bashrc
• Non-interactive shells (like with bash -c) only read files specified in BASH_ENV

Let's examine a concrete example with a Python environment:

# Case where it fails
docker exec -it py_container bash -c "python --version"
# Returns: bash: python: command not found

# Case where it works
docker exec -it py_container bash
root@py_container:/# python --version
Python 3.8.10

Here are three reliable approaches to ensure consistent environment variables:

# Method 1: Source profile explicitly
docker exec -it my_container bash -c "source ~/.bashrc && echo $PATH"

# Method 2: Use --login flag
docker exec -it my_container bash --login -c "echo $PATH"

# Method 3: Set BASH_ENV
docker exec -it -e BASH_ENV=~/.bashrc my_container bash -c "echo $PATH"

For production Docker environments, consider these architectural approaches:

1. Explicitly set required environment variables in Dockerfile
2. Use ENTRYPOINT scripts to properly initialize the environment
3. Maintain separate .bashrc for interactive and non-interactive use

# Dockerfile example
FROM ubuntu:20.04
RUN echo 'export PATH=$PATH:/custom/path' >> /etc/bash.bashrc
ENV BASH_ENV=/etc/bash.bashrc

When troubleshooting environment issues:

# Check loaded files
docker exec -it my_container bash -c "echo \$BASH_ENV; shopt -q login_shell && echo 'Login shell' || echo 'Not login shell'"

# Compare environments
docker exec -it my_container env > env_interactive.txt
docker exec -it my_container bash -c "env" > env_noninteractive.txt
diff env_*.txt

When working with Docker containers, you might encounter situations where environment variables (particularly PATH) behave differently between these two scenarios:

# Non-interactive (single command)
docker exec -i -t my_container bash -c "echo $PATH"

# Interactive session
docker exec -i -t my_container bash
root@container:/# echo $PATH

The difference stems from how bash loads configuration files:

# Interactive login shell loads:
/etc/profile
~/.bash_profile
~/.bash_login
~/.profile

# Non-interactive shell with -c only loads:
BASH_ENV (if set)

Consider this common Docker scenario where you modify PATH in .bashrc:

# Dockerfile snippet
RUN echo 'export PATH="/custom/path:$PATH"' >> ~/.bashrc

This modification won't be available in bash -c executions.

Option 1: Source the profile explicitly

docker exec -i -t my_container bash -c "source ~/.bashrc && echo $PATH"

Option 2: Use --login flag

docker exec -i -t my_container bash --login -c "echo $PATH"

Option 3: Set BASH_ENV

docker exec -i -t my_container env BASH_ENV=~/.bashrc bash -c "echo $PATH"

For consistent behavior across all execution modes:

# In Dockerfile
ENV PATH="/custom/path:${PATH}"

This ensures the PATH modification is available in all execution contexts.

To understand what's being loaded:

# Check loaded files in interactive mode
docker exec -it my_container bash -c "echo \$BASH_SOURCE"

# Check environment variables
docker exec -it my_container env