Using Variables In Bash Brace Expansion: Best Practices And Alternatives
The Problem with Unquoted Brace Expansion
Brace expansion in Bash allows convenient generation of arbitrary strings. However, unquoted brace expansions can lead to unexpected and dangerous behaviors. When brace expansions are left unquoted, the resulting strings may be improperly split into multiple arguments and improperly ordered. This can cause commands to act on the wrong files or directories, delete unintended files, or expose security vulnerabilities. Quoting brace expansion expressions is essential to avoid these pitfalls.
Why Quote Brace Expansion Expressions?
Quoting brace expansion forces Bash to preserve the expended strings as a single argument, preventing unintended splitting or reordering. Consider expanding the expression {A,B}.unquoted,{A,B} expands to the two words A and B. But quoted as “{A,B}”, the expansion remains a single argument. This prevents spliting on spaces and preserves the intended string list.
Likewise, quoting prevents reordering effects from Bash globbing and alpha-sorting. The expression {Z,a} unquoted, expands to a Z. But quoted as “{Z,a}”, the expansion preserves the intended order as a single argument. This prevents unintended reordering effects.
Using Variables to Dynamically Construct Lists
Variables can be embedded within brace expansion expressions to enable dynamic construction of lists or file patterns. However, the expansion rules remain complex, with order of operations and quoting still critical.
Example: Creating a List of Sequential Numbers
A variable can represent the starting or ending point inside a brace expansion:
$ start=1
$ end=5
$ echo {$start..$end}
1 2 3 4 5
This generates the list of numbers from 1 to 5 by expanding the {$start..$end} expression. Useful in loops for iterating over sequences of numbers.
Example: Creating a List of File Names
Likewise, variables can represent file name patterns:
$ prefix=file_
$ suffix=.txt
$ echo $prefix{A,B,C}$suffix
file_A.txt file_B.txt file_C.txt
This generates a list of files matching the pattern file_A.txt, file_B.txt, etc. Useful for operating on groups of similarly named files.
Bash Pitfalls to Avoid
While empowering, brace expansions bring complexity. Variables do not scope neatly within brace expressions. Knowledge of expansion order and quoting rules is essential to avoiding pitfalls.
Unquoted Expansions Lead to Word Splitting
Consider this incorrect attempt to use a variable expansion within braces:
$ dirs={/home /opt}
$ echo $dirs
/home /opt
Despite appearing correct, the unquoted {/home /opt} will expand into the two words /home and /opt. Adding quotes is required to preserve the intended single argument:
$ dirs="/{home,opt}"
$ echo "$dirs"
/home /opt
In the first case, failure to quote leads to word splitting. In the second case, quotes force the desired single argument expansion, preventing the word splitting.
Order of Operations Can Cause Unexpected Results
Brace expansion occurs very early, before other expansions. Consider attempting to combine brace expansion with variable expansion:
$ dirs=/h{ome,ello}
$ echo $dirs
/home /hello
Despite appearances, because brace expansion happens first, this is equivalent to:
$ dirs=/hhome /hello $ echo $dirs /hello
The $dirs variable only takes the last word. Fix this with proper quoting to force the variable expansion first:
$ dirs=/{h$phrase}
$ echo "$dirs"
/home /hello
Understanding this order of operations is key to avoiding confusion.
Alternatives to Complex Brace Expansion
In many cases, alternative Bash features can simplify tasks otherwise requiring intricate brace expansions.
Arrays for Storing Lists
Bash arrays hold lists of data over multiple array elements. Unlike brace expansion strings, arrays neatly preserve spacing, order, and quoting of elements:
$ files=(/home/first\ file /opt/second\ file)
$ echo "${files[@]}"
/home/first file /opt/second file
Accessing elements does not require substring operations. Braces access a specific element:
$ echo "${files[0]}"
/home/first file
Iteration is simplified with for loops instead of complex nesting of brace expansions.
Loops for Iterating Over Lists
Bash looping constructs can simplify sequential processing without nested brace expansion. For example, this loop echos a sequence of numbers:
$ start=0; end=5 $ for i in $(seq $start $end); do echo $i; done 0 1 2 3 4 5
The seq command generates the sequence of numbers, avoiding complex nesting of brace expansions. Loops clarify the iterated logic. Built-in Bash arrays further simplify sequentially working with lists.
Summary – When to Use Brace Expansion vs Other Tools
Brace expansion enables dynamically constructing arbitrary string lists on the Bash command line. Embedded variables provide additional flexibility to build file name patterns and sequences. However, intricate nesting of brace expansions quickly becomes complex.
Always quote brace expansion expressions to prevent splitting and reordering pitfalls from undermining logic. Be aware of Bash’s order of operations, evaluating brace and other expansions, to understand command behavior. Prefer built-in arrays and loops over complex nested brace expansions when possible.
Used properly, brace expansion can enable powerful command line list generation. But factoring tasks into scripts with better data structures like arrays generally improves readability and maintainability.
