Skip to content

Latest commit

 

History

History
218 lines (147 loc) · 3.33 KB

debugging.md

File metadata and controls

218 lines (147 loc) · 3.33 KB

Absolutely! Bash debugging is crucial to figure out why a script isn’t working as expected. Bash provides built-in options and techniques to debug scripts. I’ll give you a complete, structured guide.

📌 Debugging in Bash Scripts

1. Run Bash in Debug Mode

a) -x option

Prints each command before executing it, showing how variables are expanded.

bash -x script.sh

Example:

#!/bin/bash
NAME="Mahin"
echo "Hello $NAME"

Run with debug:

bash -x script.sh

Output:

+ NAME=Mahin
+ echo 'Hello Mahin'
Hello Mahin

b) -v option

Prints each line as it is read, before execution.

bash -v script.sh

c) -n option

Syntax check only; does not execute the script.

bash -n script.sh

2. Use set Built-in for Debugging

a) set -x / set +x

Turn on/off debug mode inside the script:

#!/bin/bash
echo "Before debug"

set -x   # Start debugging
VAR=5
echo "Value of VAR: $VAR"
set +x   # Stop debugging

echo "After debug"

b) set -e

Exit the script immediately if any command returns a non-zero status.

#!/bin/bash
set -e
echo "Step 1"
ls non_existing_file   # Script stops here
echo "Step 2"

c) set -u

Treat unset variables as an error:

#!/bin/bash
set -u
echo $UNDEFINED_VAR   # Error

d) Combine flags

set -euxo pipefail
  • -e → exit on error
  • -u → treat unset variables as errors
  • -x → print commands before executing
  • -o pipefail → fail if any command in a pipeline fails

3. Trace Execution with PS4

You can customize the prefix for -x output:

#!/bin/bash
export PS4='+ ${BASH_SOURCE}:${LINENO}:${FUNCNAME[0]}: '
set -x
myfunc() {
  echo "Inside function"
}
myfunc

Output will show file name, line number, function name for each command.

4. Debugging Loops and Functions

Use set -x locally:

#!/bin/bash
myfunc() {
  set -x
  for i in {1..3}; do
    echo "Loop $i"
  done
  set +x
}
myfunc

5. Using trap for Debugging

Run a command when an error or signal occurs.

#!/bin/bash
trap 'echo "Error at line $LINENO"' ERR
false   # triggers trap
  • $LINENO → line number where error occurred
  • $BASH_COMMAND → command being executed

Example:

trap 'echo "Command failed: $BASH_COMMAND at line $LINENO"' ERR

6. Debugging Input/Output

  • Print variables with echo or printf
  • Check if commands return expected values
  • Use set -x around suspect sections

7. Debugging Tips

  1. Use small test scripts to isolate the problem.
  2. Test loops and conditionals separately.
  3. Redirect debug output to a file:
bash -x script.sh > debug.log 2>&1
  1. Combine -x and -v for maximum verbosity.
  2. Use shellcheck (external tool) for static analysis:
shellcheck script.sh

✅ Summary

  • bash -x, set -x → trace commands
  • set -e, set -u, set -o pipefail → stop on errors
  • trap → catch errors or signals
  • PS4 → detailed debug info
  • shellcheck → static syntax and style checking

If you want, I can make a Markdown cheatsheet with all Bash debugging commands, flags, and traps, ready for reference.

Do you want me to do that?