File existence checks in Bash Scripting - Step-by-Step Execution

Concept Flow - File existence checks

Start

↓

Check if file exists

↓

End

The script checks if a file exists, then prints a message based on the result.

Execution Sample

Bash Scripting

filename="test.txt"
if [ -e "$filename" ]; then
  echo "File exists"
else
  echo "File does not exist"
fi

This script checks if 'test.txt' exists and prints a message accordingly.

Execution Table

Step	Action	Condition	Result	Output
1	Set filename to 'test.txt'	N/A	filename='test.txt'
2	Check if file exists with [ -e "$filename" ]	[ -e "test.txt" ]	True if file exists, False if not
3	If condition True	File exists?	Yes	echo 'File exists'
4	Print output	N/A	N/A	File exists
5	If condition False	File exists?	No	echo 'File does not exist'
6	Print output	N/A	N/A	File does not exist

💡 Script ends after printing the appropriate message based on file existence.

Variable Tracker

Variable	Start	After Step 1	Final
filename	undefined	test.txt	test.txt

Key Moments - 3 Insights

Why do we use quotes around "$filename" in the condition?

Quoting "$filename" prevents errors if the filename contains spaces or is empty, ensuring the condition checks the correct string as shown in step 2 of the execution_table.

What does the -e option check in the condition?

The -e option checks if the file exists regardless of type (file, directory, etc.), as seen in step 2 where the condition evaluates file existence.

Why is there an else branch in the script?

The else branch handles the case when the file does not exist, ensuring the script always prints a clear message, demonstrated in steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'filename' after step 1?

A undefined

B "test.txt"

C "filename"

D empty string

💡 Hint

Check the variable_tracker table under 'After Step 1' for 'filename'.

At which step does the script print 'File exists'?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Output' column in the execution_table for the message 'File exists'.

If the file does not exist, which step shows the printed output?

A Step 3

B Step 4

C Step 6

D Step 5

💡 Hint

Check the execution_table rows where the output is 'File does not exist'.

Concept Snapshot

File existence check in bash:
Use [ -e "$filename" ] to test if a file exists.
If true, run commands in then-block.
Else, run commands in else-block.
Always quote variables to handle spaces.
Print messages to confirm existence.

Full Transcript

This script sets a variable 'filename' to 'test.txt'. It then checks if this file exists using the condition [ -e "$filename" ]. If the file exists, it prints 'File exists'. Otherwise, it prints 'File does not exist'. Quoting the variable ensures the check works even if the filename has spaces or is empty. The script ends after printing the appropriate message.