Add chapter 025: Common Bash Errors and Fixes

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.