Master Bash Brackets: Parentheses, Brackets, and Brace Expansions Explained

1. Parentheses ()

1. Single parentheses ()

① Command grouping: commands inside parentheses run in a subshell, so variables defined there are not available after the group. Separate commands with semicolons; the last command may omit the semicolon.

② Command substitution: equivalent to $(cmd). The shell executes cmd, captures its standard output, and replaces the whole construct with that output. Some shells (e.g., tcsh) do not support this form.

③ Array initialization example: array=(a b c d) 2. Double parentheses (( ))

① Arithmetic expansion: evaluates an integer arithmetic expression; the exit status is 0 for a non‑zero result (true) and 1 for zero (false). Logical expressions follow C syntax.

② Any C‑compatible arithmetic expression can be used, including base‑prefix notation. Example: ((16#5f)) yields 95 (hex to decimal).

③ Variable increment example: a=5; ((a++)) makes a become 6.

④ Commonly used in arithmetic comparisons; variables inside do not need the $ prefix. Multiple expressions can be separated by commas, e.g., for ((i=0;i<5;i++)).

2. Square Brackets []

1. Single brackets []

① In Bash, [ is a synonym for the test builtin. It evaluates a conditional expression and returns an exit status. Modern Bash requires the closing ].

② Comparison operators: == and != for string comparison; numeric comparisons use -eq, -gt, etc. The symbols < and > are not valid for numeric tests.

③ Character class usage in regular expressions, not in test.

④ In array contexts, brackets select an element by index.

2. Double brackets [[ ]]

① [[ is a Bash keyword, not a command. Inside, pathname expansion and word splitting are suppressed, but parameter expansion and command substitution still occur.

② Supports pattern matching with =~ and globbing; patterns need not be quoted.

③ Provides safer conditional syntax; logical operators like &&, ||, <, > work without quoting issues.

④ Returns an exit status based on the evaluated expression.

Example comparisons:

if ( $i<5 )
if [ $i -lt 5 ]
if [ $a -ne 1 -a $a != 2 ]
if [ $a -ne 1 ] && [ $a != 2 ]
if [[ $a != 1 && $a != 2 ]]
for i in $(seq 0 4); do echo $i; done
for i in `seq 0 4`; do echo $i; done
for ((i=0;i<5;i++)); do echo $i; done
for i in {0..4}; do echo $i; done

3. Curly Braces { }

1. General usage

① Brace expansion (globbing): generates a list of strings. No spaces are allowed unless escaped. Examples:

# ls {ex1,ex2}.sh
ex1.sh ex2.sh
# ls {ex{1..3},ex4}.sh
ex1.sh ex2.sh ex3.sh ex4.sh
# ls {ex[1-3],ex4}.sh
ex1.sh ex2.sh ex3.sh ex4.sh

② Command grouping with braces creates an anonymous function; commands run in the current shell, so variables remain available. Commands must be separated by semicolons, and a space is required after the opening brace.

2. Parameter substitution forms

${var:-string}, ${var:+string}, ${var:=string}, ${var:?string}

• ${var:-string}: use string if var is unset or null. • ${var:+string} : use string only if var is set and non‑null. • ${var:=string} : assign string to var if it is unset or null, then expand to the new value. • ${var:?string} : if var is unset or null, print string to standard error and abort the script. 3. Pattern removal <code>${var%pattern}, ${var%%pattern}, ${var#pattern}, ${var##pattern}</code> Single % or # removes the shortest matching suffix or prefix; double %% or ## removes the longest. Examples with var=testcase : <code># var=testcase # echo ${var%s*e} # testca # echo ${var%%s*e} # te # echo ${var#?e} # stcase # echo ${var##?e} # stcase # echo ${var##*e} # (empty) # echo ${var##*s} # e # echo ${var##test}# case</code> 4. String extraction and replacement <code>${var:num}, ${var:num1:num2}, ${var/pattern/pattern}, ${var//pattern/pattern}</code> • ${var:num} : extract substring from position num to the end (positive index from the left, negative from the right). • ${var:num1:num2} : extract num2 characters starting at num1 . • ${var/pattern/pattern} : replace first occurrence of pattern . • ${var//pattern/pattern} : replace all occurrences of pattern . Example with var=/home/centos : <code># var=/home/centos # echo ${var:5} # /centos # echo ${var:-6} # centos # echo ${var:1:4} # home # echo ${var/o/h} # /hhme/centos # echo ${var//o/h} # /hhme/cenths</code> 4. Dollar‑sign expansions ① ${a} expands to the value of variable a ; braces can be omitted when unambiguous. ② $(cmd) performs command substitution, equivalent to backticks. ③ $((expression)) evaluates an arithmetic expression using C syntax, supporting ternary and logical operators. 5. Practical usage Multiple command execution examples: Single parentheses: (cmd1; cmd2; cmd3) runs in a subshell. Single braces: { cmd1; cmd2; cmd3; } runs in the current shell; a space after the opening brace is required and the final command must end with a semicolon. Redirections inside the group affect only the commands inside; redirections outside affect the whole group.