Tilde expansion (~) in Bash Scripting - Step-by-Step Execution

Concept Flow - Tilde expansion (~)

Start command with ~

↓

Detect ~ at start

↓

Replace ~ with home directory path

↓

Execute command with expanded path

↓

End

The shell detects a tilde (~) at the start of a path and replaces it with the user's home directory before running the command.

Execution Sample

Bash Scripting

echo ~
echo ~/Documents
cd ~
pwd

This script shows how ~ expands to the home directory in different commands.

Execution Table

Step	Command	Tilde Detected	Expansion Result	Output/Effect
1	echo ~	Yes	/home/username	/home/username
2	echo ~/Documents	Yes	/home/username/Documents	/home/username/Documents
3	cd ~	Yes	/home/username	Changes directory to /home/username
4	pwd	No	N/A	/home/username
5	End	N/A	N/A	Script ends

💡 All commands executed; tilde expanded to /home/username where applicable.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
PWD	/current/directory	/current/directory	/current/directory	/home/username	/home/username	/home/username

Key Moments - 2 Insights

Why does ~ expand only when it is at the start of the path?

Because tilde expansion in bash only happens when ~ is the first character of the word, as shown in steps 1-3 where ~ is at the start and expands, but not in step 4 where pwd has no ~.

What happens if ~ is used in the middle of a path?

It does not expand. For example, echo /tmp/~ would print /tmp/~ literally, because expansion only occurs at the start, as seen in the execution_table where only leading ~ expands.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of 'echo ~' at step 1?

A /home/username

B ~

C /tmp

D Error

💡 Hint

Check the 'Output/Effect' column for step 1 in the execution_table.

At which step does the current directory change to the home directory?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Output/Effect' and 'PWD' variable changes in variable_tracker after each step.

If the command was 'echo /tmp/~', what would be the output?

A /tmp/home/username

B /home/username/tmp/~

C /tmp/~

D Error

💡 Hint

Recall that tilde expands only at the start of the path, as shown in the execution_table steps.

Concept Snapshot

Tilde (~) expansion replaces ~ at the start of a path with the user's home directory.
Used in commands like cd, echo, and file paths.
Only expands if ~ is the first character of the word.
Example: ~ -> /home/username, ~/docs -> /home/username/docs.
No expansion if ~ is in the middle or end of a path.

Full Transcript

Tilde expansion in bash means the shell looks for a tilde character at the start of a path and replaces it with the user's home directory path. For example, when you run 'echo ~', the shell changes ~ to '/home/username' and prints it. Similarly, 'cd ~' changes the current directory to the home directory. This expansion only happens if the tilde is the first character in the path. If the tilde appears later in the path, it is treated as a normal character and not expanded. This behavior helps users quickly refer to their home directory in commands.