📦 Variables and Environment

Variables store values for reuse within scripts or sessions. They are essential building blocks for dynamic behavior.

🧮 Declaring and Using Variables

Assign value to variable:

name=John
age=30

Access value using $ prefix:

echo "Name: $name"
echo "Age: $age"

Enclose in braces for clarity or disambiguation:

echo "${name}Doe"   # JohnDoe

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🌐 Environment Variables

Environment variables are inherited by child processes:

export MYVAR=value
./some_script.sh     # inherits MYVAR

View all environment variables:

env
# or
printenv

Get specific one:

echo $HOME

Some commonly used ones: | Variable | Purpose | |----------|----------------------------| | HOME | User's home directory | | PATH | Directories searched for commands | | USER | Current username | | PWD | Present Working Directory |

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🧪 Parameter Expansion

Shell supports advanced expansions:

filename="/path/to/file.txt"

basename="${filename##*/}"     # file.txt
dirname="${filename%/*}"       # /path/to
extension="${filename##*.}"    # txt
without_ext="${filename%.*}"   # /path/to/file

Pattern-based replacements:

str="hello world"
new_str=${str//o/O}            # hellO wOrld

Length of string:

len=${#str}                    # 11

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🧾 Summary

• Assign variables with var=value.
• Access them with $var or ${var}.
• Export to make available to children.
• Use parameter expansion for transformations.
• Avoid spaces around = in assignments.

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