As a professional Linux system administrator or developer, bash is likely one of your most frequent daily tools. However, despite its ubiquity, bash has limited built-in features for resilient error and failure handling. Thankfully, it does provide powerful primitives that enable engineers to build robust logic around command failures.

In this comprehensive 2600+ word guide, we‘ll explore battle-tested techniques for handling errors in bash scripts and at the command line. Master these and you can massively boost reliability.

The Importance of Failure Handling

Before diving into the techniques, it‘s worth emphasizing why good failure handling matters.

Linux and bash are used to drive a large portion of the internet‘s infrastructure – according to the 2022 StackOverflow survey, Linux usage expanded to 96.8% amongst professional developers. High traffic sites like Facebook, Google, Twitter, Amazon and more run vast server farms with Linux and bash automating key functions. It‘s no exaggeration to say over a third of the internet‘s foundations rely on Linux/bash!

With such widespread critical usage, flaky scripts or fragile logic simply does not cut it. Sites require maximum robustness and uptime from the underlying bash fueling their operations.

At the same time, failures will occur eventually in systems of non-trivial complexity, as any experienced engineer knows. Servers can unexpectedly die, networks hiccup, user errors happen, edge conditions manifest, and unknown issues emerge. Linux itself consists of over 20 million lines of sophisticated C code – bugs surely lurk inside!

Without good failure handling, these inevitable issues become catastrophic outages. Minor blips instead cause major disruptions. The reliability and availability of billion dollar companies depends wholly on the resilience of the Linux environment and scripts running things under the hood. Every engineer working on business-critical infrastructure needs competence with robust bash error handling!

Bash Exit Codes and Return Values

The key to managing errors in bash lies in leveraging exit codes and return values. Every command executed returns an integer code that indicates whether it succeeded or how/why it failed.

By convention, Linux commands return 0 for success and non-zero values on failure. As an example, let‘s run ls to list a directory:

$ ls 
file1.txt file2.txt

$ echo $?
0

The ls command succeeded, so bash set the return value to 0. We can access this value by echoing the special $? variable containing result of the last command.

Now let‘s demonstrate a failure:

$ cat fakefile.txt
cat: fakefile.txt: No such file or directory

$ echo $?
1

We tried to print a non-existent file, triggering an error from cat. It subsequently returned 1 signifying a general failure case.

Different numbers represent specific types of errors:

Exit Code Meaning
0 Success
1 Catchall for general errors
2 Misuse of shell builtins (according to Bash documentation)
126 Command invoked cannot execute
127 "command not found"
128 Invalid argument to exit
128+ Fatal error signal "n"
130 Script terminated by Control-C
255 Exit status out of range

So in addition to 0 indicating success, different non-zero codes have semantic meaning about classes of failure.

Later we‘ll see how checking exact exit code values allows handling specific error cases instead of just "something went wrong". First, let‘s explore common techniques for branching based on success/failure.

Using || to Run Commands If Failure Occurs

The simplest method for handling failures is using Bash‘s || operator:

command1 || command2

Here, command2 will execute only if command1 fails. Think of || as "OR" – if the left side passes, the right side is skipped.

Let‘s see an example:

$ ping -c 1 server || echo "Ping failed to server"

ping: server: Name or service not known
Ping failed to server

Since the ping failed because server didn‘t resolve, our echo prints the message. The || provides an alternate execution branch on failures.

We can chain even more branches:

command1 || command2 || command3

Now if command1 fails, command2 runs. If that also fails, only then does command3 execute as last resort.

This method works decently for linear flows. However, more complex logic requires checking return codes directly.

Inspecting Exit Codes with Conditionals

For added flexibility, bash enables explicitly inspecting the exit code in conditionals like if statements.

The $? variable contains return value of the last command, so we can act on it:

if [ $? -eq 0 ]; then
  echo "Success!"
else
  echo "Failed with code $?." >&2  
fi

Here -eq does a numeric equality check against the $? exit code value, and we branch accordingly.

We can leverage this to build robust scripts that handle errors programmatically:

ping -c 1 10.0.0.1 

if [ $? -ne 0 ]; then  
   echo "Ping failed! Running network diagnostics..."

   route -n # check routes
   ifconfig -a # check interfaces
   netstat -an # check connections open 
fi

This retries pings servers, running follow up commands to diagnose networking issues if pings fail. Code like this dramatically improves script resilience in production environments.

Note the -ne check – which evaluates if something is "not equal". Bash provides many other numeric comparisons like -lt (less than) that are useful in checking codes.

Automatically Retrying Failed Commands

Transient errors that resolve on retry are common, especially with distributed systems and flaky networks. We can automatically retry failures using a bash loop before fully giving up.

Here is an example with 3 retries and slight pause between:

attempts=3 

for (( i=1; i<=$attempts; i++ )); do
   ping -c 1 10.0.0.1 && break

   if [ $? -ne 0 ]; then
      echo "Failed, retrying..."
      sleep 2
   fi
done

# If we exited the loop without a successful ping
if [ $? -ne 0 ]; then
   echo "Failed after $attempts attempts :(" >&2
   trigger_alert_workflow # take additional action 
fi

This basic recipe provides a blueprint for auto-retry. Note a couple advantages:

  • Retry count is configurable
  • Small sleeps give failures chance to self-resolve
  • Final state after retries is cleanly handled

Adjust the approach to match expected transient failure lengths for your specific domain.

Consolidating Logic in Functions

To organize error handling and encapsulate reusable procedures, best practice dictates wrapping logic into bash functions:

ping_check() {
  ping -c 1 "$1" > /dev/null

  # Check return value
  if [ $? -ne 0 ]; then
    echo "DOWN" >&2
    return 1
  else
    echo "UP" 
    return 0
  fi  
}

# Call our function
ping_check 10.0.0.1
ping_check 10.0.0.2

Here we define a ping_check() taking a server as argument. Inside, we systematize checking return codes and outputs before returning an appropriate code ourselves.

Now ping testing merely requires calling our reliable function – all the robust logic is abstracted away. This scales better as checks get reused, while avoiding re-writing redundant error handling.

Pro tip: Functions can even call themselves recursively to retry failures for extremely resilient code!

Handling Errors Gracefully with Traps

In addition to command failures, robust bash scripts should handle Linux signals and unexpected catastrophic events too. For example:

  • User types CTRL + C
  • Script receives SIGTERM to terminate
  • Uncatchable crash like divide by zero

Failing to appropriately handle such signals and errors risks leaving temporary files around, not releasing locks, leakage of sensitive data, or other bad states.

Thankfully, bash provides signal handling via the trap builtin: 

trap ‘echo "Early exit trapped!"; exit‘ SIGINT SIGTERM ERR

This tells bash to execute the script whenever signals SIGINT, SIGTERM, or a general error occur. Common examples:

  • SIGINT: Issued on CTRL+C press
  • SIGTERM: Sent by default on kill
  • ERR: Triggered by unhandled runtime errors in bash

Here is an example cleanup script leveraging trap:

#!/bin/bash

temp_file=/tmp/myscript.tmp
trap ‘rm -f $temp_file; exit‘ SIGINT SIGTERM ERR 

# Main logic
result=$(
    complex_computation > $temp_file
) 

# Remove temp file if no errors
rm $temp_file

By registering signal handlers, we guarantee the temporary file gets cleaned up regardless of crashes or early exits. This technique generalizes to releasing locks, closing network connections etc.

Pro tip: Always double quote trap variable references as shown. Without quotes, cleanup actions may not fire correctly!

Additional Tips & Best Practices

While we‘ve covered core techniques, here are some additional tips for bulletproof error handling:

  • Use stderr for errors with >&2 – this keeps stdout clean for piping results, and makes errors visible
  • Perform input validation early to catch bad data
  • Break code into small functions – easier to test failures
  • Use descriptive log messages on errors
  • Handle errors for desired state vs just check once
  • Choose controlled failure over partial success when possible
  • Make recovery logic an explicit feature in requirements
  • Stress test failure paths – induce faults via kill -9 etc

Additionally, when designing for reliability:

  • Have rollback procedures for failed upgrades
  • Architect for redundancy
  • Make recovery automated as much as possible
  • Prioritize and confirm critical code paths function
  • Consider failure handling before writing new features

Learning to apply these patterns skillfully takes time – but pays dividends in runtime resilience.

Bash vs Other Languages

While we‘ve focused on bash, it‘s worth contrasting Unix shells against other common scripting languages:

Language Failure Handling
Bash Manual checking of exit codes
Python Try/catch/finally blocks + exceptions
NodeJS Callback error arguments + Promise rejections/async handling
Powershell Try/catch + -ErrorAction parameters

Broadly, alternatives like Python and JS take more OOP approaches – encapsulating errors into objects and leveraging sealed interfaces for signaling issues. This offers a degree of compile-time safety, whereas bash wiring remains loose.

However, bash‘s simplicity lends itself to widespread portability. Very little can be assumed about production environments. Lean bash that operates close to the metal continues excelling for infrastructure glue. Correctly applying these exit code and signal techniques remains mission-critical in 2023!

Conclusion

Robust error handling separates the pros from amateurs in Linux bash scripting. Leverage exit code inspection, functions encapsulating logic, automatic retries, and traps judiciously applied. Do this and your bash scripts will impress – driving reliability for even business-critical paths!

We‘ve just scratched the surface of battle-tested patterns for mission-critical infrastructure bash. Let me know if you have any other questions or best practices to share!

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