double-bracket

[[ expression ]]

[[ is bash/zsh's enhanced conditional expression. It provides safer and more powerful tests than [.Key advantages over [:- No word splitting on variables (unquoted $var is safe)- Pattern matching with == and !=- Regular expression matching with =~- && and || work inside the brackets- < and > don't need escaping

$ # Pattern matching
[[ $file == *.txt ]] && echo "text file"

# Regex matching
if [[ $email =~ ^[A-Za-z]+@[A-Za-z]+\.[A-Za-z]+$ ]]; then
    echo "Valid email format"
fi

# No quoting needed
[[ $var == "test" ]]  # Safe even if var is empty

File tests: Same as [ (-f, -d, -e, -r, -w, -x, etc.)String comparison:- == or =: Pattern match (glob)- !=: Not pattern match- =~: Regex match- <, >: Lexicographic orderNumeric: -eq, -ne, -lt, -le, -gt, -geLogical:- &&: AND- ||: OR- !: NOT

[[ is a reserved word in bash, zsh, and ksh, but it is not POSIX. Scripts with a `#!/bin/sh` shebang must use [ instead, since `/bin/sh` is `dash` on Debian and Ubuntu and will fail with a syntax error.The quoting rules are inverted from what people expect, and this is the most common source of bugs:Quote the right-hand side of == to compare literally. Unquoted, it is a glob pattern, so `[[ $x == pattern* ]]` matches a prefix while `[[ $x == "pattern*" ]]` matches the literal asterisk.Do not quote a regex after =~. Quoting turns the pattern into a literal string, so `[[ $x =~ "^test" ]]` looks for the characters `^test` rather than anchoring the match. Store the pattern in a variable if it needs to contain spaces.The regex dialect is ERE, and captured groups land in the `BASH_REMATCH` array, with the whole match in element 0.< and > compare strings lexicographically, not numerically: `[[ 10 < 9 ]]` is true. Use `((` `))` or the `-lt`/`-gt` operators for numbers.

[[ originated in the Korn shell (ksh88), where David Korn added it to fix the long-standing traps in the `test` command: `[` is an ordinary command, so its arguments are subject to word splitting and glob expansion, which is why an empty or unset variable turns `[ $x = y ]` into a syntax error. Making the construct a reserved word instead let the shell parse it before expansion, which is exactly what removes the need for defensive quoting.bash adopted it in version 2.02 (1998), zsh has it as well, and it is now standard practice in scripts that are not required to be POSIX. It was proposed for POSIX but rejected, so `[` remains the only portable option.

test(1), bash(1), zsh(1), ksh(1), double-parenthesis(1)