Error logging patterns in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of error logging in bash scripts?

Error logging helps capture and record errors during script execution. It makes troubleshooting easier by showing what went wrong and where.

beginner

How do you redirect error messages to a log file in bash?

Use the syntax command > logfile 2>&1. This sends both standard output and error output to the log file.

intermediate

What does 2>&1 mean in bash redirection?

It means redirect standard error (file descriptor 2) to the same place as standard output (file descriptor 1).

intermediate

Why use a function for error logging in bash scripts?

A function centralizes error handling. It makes the script cleaner and easier to maintain by reusing the same logging format and actions.

intermediate

Show a simple bash function that logs errors with timestamps.

Example:

log_error() {
  echo "$(date '+%Y-%m-%d %H:%M:%S') ERROR: $1" >&2
}

This prints the error message with a timestamp to standard error.

Which bash redirection sends both output and errors to the same file?

A command > logfile 2>&1

B command 2> logfile

C command > logfile

D command 1> logfile 1>&2

Option A redirects standard output to logfile and then redirects standard error (2) to the same place as standard output (1).

What does the following bash code do? exec 2>error.log

A Redirects standard output to error.log

B Redirects all future error messages to error.log

C Deletes error.log

D Runs error.log as a command

The exec command changes the file descriptor 2 (standard error) to write into error.log for the rest of the script.

Why is it helpful to include timestamps in error logs?

A To confuse users

B To make logs bigger

C To speed up the script

D To know when errors happened

Timestamps help track when errors occurred, which is useful for debugging and correlating events.

Which of these is a good practice for error logging in bash scripts?

A Logging errors with clear messages and timestamps

B Ignoring errors silently

C Printing errors only to the screen without saving

D Using random file names for logs

Clear messages with timestamps make error logs useful for troubleshooting.

What does this function do?

log_error() {
  echo "ERROR: $1" >&2
}

A Deletes error messages

B Prints an error message to standard output

C Prints an error message to standard error

D Logs errors to a file automatically

The function prints the message passed as $1 to standard error using >&2.

Explain how to redirect error messages to a log file in a bash script and why it is useful.

Think about how to capture errors and save them for later review.

Describe how you would create a reusable error logging function in bash and what features it should have.

Consider how to make error messages clear and easy to find.

Practice

1. What does the 2> operator do in a bash script?

easy

A. Redirects error messages to a file

B. Redirects standard output to a file

C. Appends output to a file

D. Runs a command in the background

Solution

1. Step 1: Understand bash output streams

   Bash has two main output streams: standard output (1) and standard error (2).

2. Step 2: Identify the meaning of 2>

   The operator 2> redirects the standard error stream to a file or location.

3. Final Answer:

   Redirects error messages to a file -> Option A

4. Quick Check:

   2> means error redirect [OK]

Hint: Remember 2 is error stream, so 2> redirects errors [OK]

Common Mistakes:

• Confusing 2> with > which redirects standard output
• Thinking 2> appends instead of overwriting
• Assuming 2> runs commands in background

2. Which of the following is the correct syntax to redirect only error messages (overwriting the file) to a file named error.log?

easy

A. command > error.log

B. command 2>> error.log

C. command &> error.log

D. command 2> error.log

Solution

1. Step 1: Understand redirection operators

   2> redirects standard error and overwrites the file; 2>> appends errors.

2. Step 2: Match requirement to syntax

   We want to redirect only errors and overwrite the file, so 2> error.log is correct.

3. Final Answer:

   command 2> error.log -> Option D

4. Quick Check:

   Use 2> for error overwrite [OK]

Hint: Use 2> to overwrite error file, 2>> to append [OK]

Common Mistakes:

• Using > which redirects standard output, not errors
• Using &> which redirects both output and errors
• Confusing append (>>) with overwrite (>)

3. What will be the output of the following command if script.sh prints "Success" to standard output and "Error occurred" to standard error?

./script.sh > output.log 2> error.log

medium

A. "Success" is saved in output.log, "Error occurred" in error.log

B. Both messages saved in output.log

C. Both messages appear on the terminal

D. "Error occurred" saved in output.log, "Success" in error.log

Solution

1. Step 1: Analyze redirection operators

   > output.log redirects standard output to output.log, 2> error.log redirects standard error to error.log.

2. Step 2: Match outputs to files

   "Success" goes to standard output, so saved in output.log; "Error occurred" goes to standard error, saved in error.log.

3. Final Answer:

   "Success" is saved in output.log, "Error occurred" in error.log -> Option A

4. Quick Check:

   Separate output and error files [OK]

Hint: Use > for output, 2> for errors to separate logs [OK]

Common Mistakes:

• Assuming both outputs go to same file
• Confusing 2> with >
• Expecting output on terminal despite redirection

4. Identify the error in this command intended to redirect errors to errors.log:

command 2>2 errors.log

medium

A. Should use &> instead of 2>

B. Missing space between command and 2>

C. Extra '2' causes errors to be redirected to file named '2' instead of 'errors.log'

D. No error, command is correct

Solution

1. Step 1: Examine the redirection syntax

   The correct syntax to redirect errors is 2> filename. Here, 2>2 errors.log redirects stderr to file 2, treating errors.log as a command argument.

2. Step 2: Identify the cause of error

   The extra '2' makes the filename 2 instead of errors.log.

3. Final Answer:

   Extra '2' causes errors to be redirected to file named '2' instead of 'errors.log' -> Option C

4. Quick Check:

   2> must be followed by correct filename [OK]

Hint: Check redirection target filename after 2> [OK]

Common Mistakes:

• Adding extra numbers or symbols in filename position
• Confusing &> with 2>
• Missing spaces causing parsing errors

5. You want to run a script and save both standard output and error messages into separate files, but also keep the error messages appended (not overwritten). Which command achieves this?

hard

A. ./script.sh > output.log 2> error.log

B. ./script.sh > output.log 2>> error.log

C. ./script.sh &> output.log

D. ./script.sh >> output.log 2> error.log

Solution

1. Step 1: Understand output and error redirection

   > overwrites standard output, 2>> appends standard error to a file.

2. Step 2: Match requirement to command

   We want output overwritten in output.log and errors appended in error.log, so ./script.sh > output.log 2>> error.log fits.

3. Final Answer:

   ./script.sh > output.log 2>> error.log -> Option B

4. Quick Check:

   Use 2>> to append errors [OK]

Hint: Use 2>> to append errors, > to overwrite output [OK]

Common Mistakes:

• Using 2> which overwrites error file
• Using &> which merges output and errors
• Appending output but overwriting errors