Error Handling in Bash

Since bash operates on exit codes for conditionals, you can easily use that to your advantage in error handling.

Basic Conditional Error Handling

Using if Statements

A typical way to handle errors is with if statments.

if ! sudo apt install -y someprogram; then
    printf "[ERROR]: Failed to install program!\n"
fi

Since conditionals operate on exit codes, the apt install command will run, then the if statement will check the exit code of that command.

If the exit code of apt install is non-zero, then we enter into the code block, and the printf command will run.

If the exit code is zero, the if check will not pass and the program will continue to execute normally.

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Using $?

The $? variable is a special variable in Bash that stores the exit code of the last run command.

This is also combined with if statements.

sudo apt install -y someprogram
if [[ $? -ne 0 ]]; then
    printf "[ERROR]: Failed to install package!\n"
fi

This works much the same as the previous if ! cmd..., but this gives us the option to check for specific exit codes.

For instance, if we know grep will exit with 1 if no lines were selected, and 2 if there was an error, and we just want to know if no lines were selected, we could check for an exit code of 1.

grep -qi 'term' /some/file.txt
if [[ $? -eq 1 ]]; then
    printf "[ERROR]: Line was not found!\n"
fi

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Using Inline Conditional Logic

Another method of error handling is to use an inline || (OR):

sudo apt install -y someprogram || printf "[ERROR]: Failed to install program!\n"

• The || here will only be triggered if the exit code of apt install is non-zero.

Keep in mind, when using ||, you can only specify one command to reliably run.

Doing this won't work as intended:

# BAD:
sudo apt install -y someprogram || printf "[ERROR]: Failed to install program!\n" && exit 1 

This could potentially run the exit 1 command unintentionally due to how bash handles conditional logic execution.

The way around this is to use a command group ({ ... }):

sudo apt install -y someprogram || {
    printf "[ERROR]: Failed to install program!\n" && exit 1
}

This will only run the command group if the first command (apt install) fails.