Error logging patterns in Bash Scripting - Step-by-Step Execution

Concept Flow - Error logging patterns

Start Script

↓

Run Command

↓

Check Exit Status

↓

Log Error

↓

End Script

The script runs a command, checks if it failed, logs an error if so, then continues or ends.

Execution Sample

Bash Scripting

command_that_might_fail
if [ $? -ne 0 ]; then
  echo "Error: command failed" >> error.log
fi

Runs a command, checks if it failed, and logs an error message to a file if it did.

Execution Table

Step	Action	Command Exit Status ($?)	Condition	Log Action	Output
1	Run command_that_might_fail	1	1 != 0 (True)	Append error message to error.log	error.log updated with 'Error: command failed'
2	Check condition again	1	No further action	No log	No change
3	End script	-	-	-	Script ends

💡 Command exit status is 1 (failure), so error is logged and script ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
$?	-	1	1	1
error.log	empty or previous content	added error message	unchanged	contains error message

Key Moments - 3 Insights

Why do we check if $? is not zero?

Because $? holds the exit status of the last command; zero means success, non-zero means failure. See execution_table step 1 where $? is 1, triggering the error log.

Does the error message overwrite or append to the log file?

It appends using >>, so previous logs stay. This is shown in execution_table step 1 where the message is added to error.log.

What happens if the command succeeds?

If $? is 0, the condition is false and no error is logged. The script continues without changes to error.log, as shown by the absence of logging in execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $? after running the command?

A 0

B -1

C 1

D Undefined

💡 Hint

Check the 'Command Exit Status ($?)' column in step 1 of the execution_table.

At which step is the error message added to error.log?

A Step 1

B Step 2

C Step 3

D No step logs error

💡 Hint

Look at the 'Log Action' column in the execution_table.

If the command exit status was 0, how would the execution_table change?

A Error message would still be logged

B No error message would be logged

C Script would exit immediately

D Command would run again

💡 Hint

Refer to the 'Condition' and 'Log Action' columns in the execution_table for when $? equals 0.

Concept Snapshot

Error logging in bash:
Run a command
Check exit status with $? (0=success, non-zero=failure)
If failure, append error message to a log file
Use '>>' to keep previous logs
This helps track failures without stopping script

Full Transcript

This visual execution shows how a bash script runs a command, checks its exit status using $?, and logs an error message to a file if the command fails. The flow starts with running the command, then checking if $? is not zero, which means failure. If failed, the script appends an error message to error.log. Variables tracked include $? and the contents of error.log. Key points include understanding $? exit codes, appending logs instead of overwriting, and no logging on success. The quiz tests understanding of exit status, logging step, and behavior on success.