Add chapter 025: Common Bash Errors and Fixes by Nishika10 · Pull Request #189 · bobbyiliev/introduction-to-bash-scripting

286 changes: 286 additions & 0 deletions ebook/en/content/025-common-bash-errors-and-fixes.md

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	# Common Bash Errors and Fixes

	Bash scripting is widely used for automation in Linux, but beginners often face errors that can be confusing.
	This chapter explains common Bash errors, why they occur, and how to fix them.
	Each explanation includes examples to make it easy to understand and apply in real scenarios.

	By learning these common mistakes, you can debug scripts faster, write cleaner code, and avoid repeated errors.


	## 1. Permission Denied
	### Error:
	```bash
	bash: ./script.sh: Permission denied
	```
	### Cause:

	The script file is not executable, which prevents the shell from running it.

	### Fix:
	Grant execute permission using:
	```bash
	chmod +x script.sh
	```
	Then run the script:
	```bash
	./script.sh
	```
	### Example:
	```bash
	#!/bin/bash
	echo "Hello, world!"
	```
	Without `chmod +x`, running the script results in an error.
	After granting execute permission, the script runs successfully.

	Explanation: Linux requires explicit permission to run scripts for security reasons. Always set execute permission before running a new script.


	## 2. Bad Interpreter or No Such File
	### Error:
	```bash
	bash: ./script.sh: bad interpreter: /bin/bash^M: No such file or directory
	```
	### Cause:

	This occurs when a script is created on Windows. Windows uses carriage return characters (`\r\n`) which Linux cannot read.

	### Fix:

	Convert the file to Unix format:
	```bash
	dos2unix script.sh
	```
	Or recreate the file using a Linux text editor such as nano or vim.

	Explanation: The first line of a script (`#!/bin/bash`) tells the shell which interpreter to use. Extra Windows characters break this line, causing the error.

	## 3. Command Not Found
	### Error:
	```bash
	bash: myscript: command not found
	```
	### Cause:
	The shell cannot locate the command or script. It might not be in the system PATH or is executed without specifying the path.

	### Fix:
	Run the script from its current directory:
	```bash
	./myscript.sh
	```
	Or add its directory to PATH:
	```bash
	export PATH=$PATH:/path/to/script
	```

	Explanation: The shell searches for commands in directories listed in `$PATH`. Scripts outside these directories require an explicit path.

	## 4. Syntax Error Near Unexpected Token
	### Error:
	```
	syntax error near unexpected token `then'
	```
	### Cause:

	A missing `then`, semicolon (`;`) , or incorrect control structure causes Bash to fail parsing the script.

	### Fix:

	Ensure correct syntax for conditional statements.

	Incorrect:
	```bash
	if [ $num -gt 10 ]
	echo "Number greater than 10"
	fi
	```

	Correct:
	```bash
	if [ $num -gt 10 ]; then
	echo "Number greater than 10"
	fi
	```

	Explanation: Bash requires a specific structure for `if` statements: `[condition]; then` followed by commands and closed with `fi`.


	## 5. Unexpected End of File (EOF)
	### Error:
	```
	syntax error: unexpected end of file
	```
	### Cause:

	A block (like `if`, `for`, or `while`) or a quote is not properly closed.

	### Example:

	Incorrect:
	```bash
	if [ $num -gt 5 ]; then
	echo "Greater"
	```

	Correct:
	```
	if [ $num -gt 5 ]; then
	echo "Greater"
	fi
	```

	Explanation: Every opening keyword like `if` or `for` must have a corresponding closing keyword (`fi, done`) for Bash to understand the block.


	## 6. Variable Not Expanding
	## Issue:

	A variable prints empty or does not display its value.

	### Cause:

	The variable is not initialized or not exported correctly.

	### Fix:
	```bash
	username="Nishika"
	echo "User is $username"
	```

	Explanation: Variables must be assigned a value before usage. Otherwise, Bash prints nothing.

	## 7. Integer Expression Expected
	### Error:
	```bash
	[: 5a: integer expression expected
	```
	### Cause:

	A non-numeric value is used in an arithmetic comparison.

	### Fix:
	```bash
	num=5
	if [ $num -gt 3 ]; then
	echo "Yes"
	fi
	```

	Explanation: Bash arithmetic comparisons work only with integers. Make sure the variable contains a number.

	## 8. Bad Substitution
	### Error:
	```bash
	bad substitution
	```
	### Cause:

	Bash-specific syntax is used in a shell that does not support it (like `sh`).

	### Fix:

	Run the script with Bash explicitly:
	```bash
	bash script.sh
	```

	Explanation: Not all shells support Bash extensions. Always use Bash when using advanced syntax.


	## 9. File Redirection Permission Denied
	### Error:
	```bash
	bash: output.txt: Permission denied
	```
	### Cause:

	The user does not have permission to write to the file or directory.

	### Fix:

	Redirect output to a writable location:
	```bash
	./script.sh > ~/output.txt
	```

	Or run with elevated permissions:
	```bash
	sudo ./script.sh
	```

	Explanation: Linux enforces strict file permissions. Writing to protected directories requires proper rights.


	## 10. Script Not Found
	### Error:
	```bash
	bash: script.sh: command not found
	```
	### Cause:

	The script is executed without specifying the relative or absolute path.

	### Fix:
	```bash
	./script.sh
	```

	Or provide full path:
	```bash
	/path/to/script.sh
	```

	Explanation: Bash will only execute scripts it can locate. Use relative or absolute paths when needed.

	## Practical Example: Loop Over Files

	Beginners often write loops that process a directory itself instead of the files inside it. This is a logical mistake that runs without a syntax error but produces unexpected results.

	Incorrect:
	```bash
	for file in /home/user/docs
	do
	echo Processing $file
	done
	```

	### Output:
	```bash
	Processing /home/user/docs
	```
	Correct:
	```bash
	for file in /home/user/docs/*; do
	echo "Processing $file"
	done
	```

	Explanation: Adding `/*` ensures the loop iterates over each file inside the directory, rather than the directory itself.

	## Debugging Tips

	- `set -x` – Display each command before execution.

	- `$?` – Check the exit status of the last command.

	- `bash -n script.sh` – Validate syntax without running the script.

	- `bash -v script.sh` – Display commands as they execute.

	- `2> error.log` – Redirect errors to a file.

	- `trap 'echo "Error on line $LINENO"' ERR` – Catch runtime errors with line numbers.

	## Key Takeaways

	Most common Bash errors result from:

	- Missing permissions

	- Syntax mistakes

	- Unset variables

	- Running scripts with the wrong shell

	Understanding these errors and using debugging techniques will help you write reliable and maintainable Bash scripts.

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