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[ vs [[

This answer will cover the [ vs [[ subset of the question.

Some differences on Bash 4.3.11:

• POSIX vs Bash extension:

  • [ is POSIX
  • [[ is a Bash extension documented at: https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs

• regular command vs magic

  • [ is just a regular command with a weird name.

    ] is just an argument of [ that prevents further arguments from being used.

  Ubuntu 16.04 actually has an executable for it at /usr/bin/[ provided by coreutils, but the bash built-in version takes precedence.

  Nothing is altered in the way that Bash parses the command.

  In particular, < is redirection, && and || concatenate multiple commands, ( ) generates subshells unless escaped by \, and word expansion happens as usual.

  • [[ X ]] is a single construct that makes X be parsed magically. <, &&, || and () are treated specially, and word splitting rules are different.

    There are also further differences like = and =~.

  In Bashese: [ is a built-in command, and [[ is a keyword: https://askubuntu.com/questions/445749/whats-the-difference-between-shell-builtin-and-shell-keyword

• <

  • [[ a < b ]]: lexicographical comparison
  • [ a \< b ]: Same as above. \ required or else does redirection like for any other command. Bash extension.
  • I could not find a POSIX alternative to this, see: https://stackoverflow.com/questions/21294867/how-to-test-strings-for-less-than-or-equal

• && and ||

  • [[ a = a && b = b ]]: true, logical and
  • [ a = a && b = b ]: syntax error, && parsed as an AND command separator cmd1 && cmd2
  • [ a = a -a b = b ]: equivalent, but deprecated by POSIX
  • [ a = a ] && [ b = b ]: POSIX recommendation

• (

  • [[ (a = a || a = b) && a = b ]]: false
  • [ ( a = a ) ]: syntax error, () is interpreted as a subshell
  • [ \( a = a -o a = b \) -a a = b ]: equivalent, but () is deprecated by POSIX
  • ([ a = a ] || [ a = b ]) && [ a = b ] POSIX recommendation

• word splitting

  • x='a b'; [[ $x = 'a b' ]]: true, quotes not needed
  • x='a b'; [ $x = 'a b' ]: syntax error, expands to [ a b = 'a b' ]
  • x='a b'; [ "$x" = 'a b' ]: equivalent

• =

  • [[ ab = a? ]]: true, because it does pattern matching (* ? [ are magic). Does not glob expand to files in current directory.
  • [ ab = a? ]: a? glob expands. So may be true or false depending on the files in the current directory.
  • [ ab = a\? ]: false, not glob expansion
  • = and == are the same in both [ and [[, but == is a Bash extension.
  • printf 'ab' | grep -Eq 'a.': POSIX ERE equivalent
  • [[ ab =~ 'ab?' ]]: false, loses magic with ''
  • [[ ab? =~ 'ab?' ]]: true

• =~

  • [[ ab =~ ab? ]]: true, POSIX extended regular expression match, ? does not glob expand
  • [ a =~ a ]: syntax error. No bash equivalent.
  • printf 'ab' | grep -Eq 'ab?': POSIX equivalent

Recommendation: always use [].

There are POSIX equivalents for every [[ ]] construct I've seen.

If you use [[ ]] you:

• lose portability
• force the reader to learn the intricacies of another bash extension. [ is just a regular command with a weird name, no special semantics are involved.