- 「[ 」は外部コマンド。/usr/bin 以下にある実行可能ファイル。「 [ 」がコマンド本体で「 ] 」はパラメータ。 「 [ 」の直後にスペースがないとエラーになるのはそのため。
$ which [ /usr/bin/[ $ ls -l /usr/bin/[ -rwxr-xr-x 1 root root 31048 Sep 4 07:09 /usr/bin/[ $ file /usr/bin/[ /usr/bin/[: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.6.9, dynamically linked (uses shared libs), for GNU/Linux 2.6.9, stripped
- 「[ [」はbash の組み込みコマンド
$ which [[ /usr/bin/which: no [[ in (/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/home/neo/bin) $ man bash ... [[ expression ]] Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as -f must be unquoted to be recognized as primaries. When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below under Pattern Matching. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic charac- ters. The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string. An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression’s return value is 2. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. Substrings matched by parenthesized subexpres- sions within the regular expression are saved in the array variable BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expression. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthe- sized subexpression. Expressions may be combined using the following operators, listed in decreasing order of precedence: ( expression ) Returns the value of expression. This may be used to override the normal precedence of operators. ! expression True if expression is false. expression1 && expression2 True if both expression1 and expression2 are true. expression1 || expression2 True if either expression1 or expression2 is true. The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire condi- tional expression.
なーるほど。
