Brace Expansion, Variable Expansion And Evaluation Order In Bash: Demystified

Bash scripting provides immense power and flexibility for automating tasks and workflows on Linux systems. However, this power comes with complexity around order of operations. Understanding how Bash handles brace expansion, variable expansion, and evaluation order will help you write clearer, more robust Bash scripts.

Understanding Brace Expansion

Brace expansion allows generating arbitrary strings for usage in paths, filenames, and more. It follows a well-defined syntax and set of usage rules.

Definition and Syntax of Brace Expansion

Brace expansion refers to the ability to generate multiple text strings from a pattern containing braces {}. Some key properties:

• Expressions take the form {x1..x2}, where x1 and x2 are either literals or integers.
• Integer sequences generate numeric strings from the range x1 to x2.
• Literal characters are taken as discrete textual elements.
• Nested braces and comma-delimited lists are allowed.

For example, the expression {a..c} would expand to the three words “a”, “b”, “c”. The expression {1..5} would expand to “1”, “2”, “3”, “4”, “5”.

Examples of Common Brace Expansion Use Cases

Brace expansions are useful in situations that call for systematic naming or enumeration:

• Filenames – Generate file sequence such as log001.txt, log002.txt, etc.
• Directories – Create directories for Jan, Feb, Mar and so on based on month numbers.
• Arrays – Populate a Bash array with integers from 1 up to some MAXNUM value.
• Options – Expand common option flags like {a..d} into -a, -b, -c, or -d.

Brace expansion translations happen early in Bash’s order of evaluation before most other expansions.

Expanding Variables in Bash

Like most languages, Bash allows variables to store data values for reference elsewhere. Bash has unique rules regarding typical and special variables.

How Bash Expands Variables

When Bash encounters a variable reference like $FOO, it tries to replace it with the variable’s value in that scope:

• $FOO becomes the textual contents stored in variable FOO.
• Unset variables expand to empty strings.
• The $ triggers the variable substitution.
• Names consist alphanumeric values plus underscore.
• Curly braces {} can be used to explicitly scope variable names.

Special Parameters like $0, $#, $@ etc.

Bash defines special variable parameters that refer to details about the running script itself:

• $0 – Name of current script
• $1 – $9 – Arguments passed to script or function
• $@ – All arguments as discrete words
• $# – Number of arguments
• $$ – Process ID (PID) of the script
• $? – Exit status of last run command

These intrinsic Bash variables provide easy access to metadata about the ongoing execution and flow control state.

Examples of Using Variable Expansion

Variable expansions are pervasive in Bash. Some usage examples include:

• Include filename prefix: logfile=$LOG_DIR/app-$DATE.log
• Reference script arguments: echo "First arg: $1"
• Get script name dynamically: name=$(basename $0)
• Enumerate arguments: for arg in "$@"; do ... ; done

As with many languages, variables form the backbone of dynamic behavior in Bash allowing flexible reuse of state values.

Evaluation Order in Bash

Understanding the exact order Bash evaluates expansions and commands helps explain script behavior. Evaluations adhere to distinct precedence rules.

Operator Precedence and Evaluation Order

Bash broadly follows this order when evaluating expressions:

1. Brace expansion
2. Variable assignment
3. Function execution
4. Variable expansion such as $FOO
5. Logical operators like && and ||
6. Mathematical evaluations in $(( )) blocks
7. File tests with [, [[, and conditional commands

Knowing this evaluation sequence enables clearly interpreting complex logical tests and behavior.

Examples of How Evaluation Order Impacts Results

Evaluation directives like set -e (exit on errors) apply after mathematical tests. Odd results can occur in nest conditional tests:

if [[ $(( 1 / 0 )) -gt 1 ]]; then 
   echo "Error bypassed due to eval order"
fi

The above errors out from the division, but the test still runs to completion since the math is done first inline. Always simplicity logical tests.

Likewise variable assignment, expansion, and export happen inline during evaluation:

if MYVAR=hello; then
   echo $MVAR   # No output!
   export MYVAR # Fails silently
fi
# MYVAR is unset after logic test finishes 

Understanding exactly how Bash evaluates complex expressions helps explain and eliminate quirky behaviors in scripts.

Putting It All Together

Brace expansions, variable assignments, and Bash evaluation orders combine in intricate ways. Some examples illustrate the composite behavior.

Complex Examples with Brace Expansion, Variable Expansion, and Evaluation Order

Nesting variable references inside brace expansion generates interesting combinations:

# Enumerate arguments
for i in {1..$#}; do
   echo "Arg $i = ${!i}" 
done

The brace expansion {1..$#} first generates a sequence from 1 to the argument count. This feeds the loop iterator. Inside, argument index variable indirection fettes each arg value in one pass.

Best Practices for Readable and Maintainable Bash Scripts

Some tips for harnessing Bash syntax cleanly include:

• Break complex one-liners into distinct testable steps
• Use functions to encapsulate logic reusal
• Flatten nested expansions for readability when possible
• Reference precedents with inline comments
• Structure logic flow control cleanly in if/else constructs

Bash grants intricate power through its evaluation syntax. Structuring scripts methodically helps smooth complexity to best leverage this capacity.