今天发现的Mac的bin/目录下有个奇怪的文件，文件名叫‘[’，那这个文件是用来干嘛的呢？尝试用Mach-o看过内部的Symbol Table，看到一些比较相关的方法：
搜索文章发现：
https://apple.stackexchange.com/questions/266620/what-is-the-bin-file-in-os-x
看来早有人已经发现了这个问题，执行：
man [
TEST(1) BSD General Commands Manual TEST(1)
NAME
test, [ -- condition evaluation utility
SYNOPSIS
test expression
[ expression ]
DESCRIPTION
The test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; otherwise it returns 1 (false). If there is no expression, test also returns 1 (false).
All operators and flags are separate arguments to the test utility.
The following primaries are used to construct expression:
-b file True if file exists and is a block special file.
-c file True if file exists and is a character special file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID flag is set.
-h file True if file exists and is a symbolic link. This operator is retained for compatibility with previous versions of this program. Do not rely on its existence; use -L instead.
-k file True if file exists and its sticky bit is set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
-t file_descriptor
True if the file whose file descriptor number is file_descriptor is open and is associated with a terminal.
-u file True if file exists and its set user ID flag is set.
-w file True if file exists and is writable. True indicates only that the write flag is on. The file is not writable on a read-only file system even if this test indicates true.
DESCRIPTION
The test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; otherwise it returns 1 (false). If there is no expression, test also returns 1 (false).
All operators and flags are separate arguments to the test utility.
The following primaries are used to construct expression:
-b file True if file exists and is a block special file.
-c file True if file exists and is a character special file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID flag is set.
-h file True if file exists and is a symbolic link. This operator is retained for compatibility with previous versions of this program. Do not rely on its existence; use -L instead.
-k file True if file exists and its sticky bit is set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
-t file_descriptor
True if the file whose file descriptor number is file_descriptor is open and is associated with a terminal.
-u file True if file exists and its set user ID flag is set.
-w file True if file exists and is writable. True indicates only that the write flag is on. The file is not writable on a read-only file system even if this test indicates true.
-x file True if file exists and is executable. True indicates only that the execute flag is on. If file is a directory, true indicates that file can be searched.
-z string True if the length of string is zero.
-L file True if file exists and is a symbolic link.
-O file True if file exists and its owner matches the effective user id of this process.
-G file True if file exists and its group matches the effective group id of this process.
-S file True if file exists and is a socket.
file1 -nt file2
True if file1 exists and is newer than file2.
file1 -ot file2
True if file1 exists and is older than file2.
file1 -ef file2
True if file1 and file2 exist and refer to the same file.
string True if string is not the null string.
s1 = s2 True if the strings s1 and s2 are identical.
s1 != s2 True if the strings s1 and s2 are not identical.
s1 < s2 True if string s1 comes before s2 based on the binary value of their characters.
s1 > s2 True if string s1 comes after s2 based on the binary value of their characters.
n1 -eq n2 True if the integers n1 and n2 are algebraically equal.
n1 -ne n2 True if the integers n1 and n2 are not algebraically equal.
n1 -gt n2 True if the integer n1 is algebraically greater than the integer n2.
n1 -ge n2 True if the integer n1 is algebraically greater than or equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less than the integer n2.
n1 -le n2 True if the integer n1 is algebraically less than or equal to the integer n2.
If file is a symbolic link, test will fully dereference it and then evaluate the expression against the file referenced, except for the -h and -L primaries.
These primaries can be combined with the following operators:
! expression True if expression is false.
TEST(1) BSD General Commands Manual TEST(1)
NAME
test, [ -- condition evaluation utility
TEST(1) BSD General Commands Manual TEST(1)
NAME
test, [ -- condition evaluation utility
SYNOPSIS
test expression
[ expression ]
DESCRIPTION
The test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; otherwise it returns 1 (false). If there is no expression, test also returns 1 (false).
All operators and flags are separate arguments to the test utility.
The following primaries are used to construct expression:
-b file True if file exists and is a block special file.
-c file True if file exists and is a character special file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID flag is set.
-h file True if file exists and is a symbolic link. This operator is retained for compatibility with previous versions of this program. Do not rely on its existence; use -L instead.
-k file True if file exists and its sticky bit is set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
-t file_descriptor
True if the file whose file descriptor number is file_descriptor is open and is associated with a terminal.
-u file True if file exists and its set user ID flag is set.
-w file True if file exists and is writable. True indicates only that the write flag is on. The file is not writable on a read-only file system even if this test indicates true.
-x file True if file exists and is executable. True indicates only that the execute flag is on. If file is a directory, true indicates that file can be searched.
-z string True if the length of string is zero.
-L file True if file exists and is a symbolic link.
-O file True if file exists and its owner matches the effective user id of this process.
-G file True if file exists and its group matches the effective group id of this process.
-S file True if file exists and is a socket.
file1 -nt file2
True if file1 exists and is newer than file2.
file1 -ot file2
True if file1 exists and is older than file2.
file1 -ef file2
True if file1 and file2 exist and refer to the same file.
string True if string is not the null string.
s1 = s2 True if the strings s1 and s2 are identical.
s1 != s2 True if the strings s1 and s2 are not identical.
s1 < s2 True if string s1 comes before s2 based on the binary value of their characters.
s1 > s2 True if string s1 comes after s2 based on the binary value of their characters.
n1 -eq n2 True if the integers n1 and n2 are algebraically equal.
n1 -ne n2 True if the integers n1 and n2 are not algebraically equal.
n1 -gt n2 True if the integer n1 is algebraically greater than the integer n2.
n1 -ge n2 True if the integer n1 is algebraically greater than or equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less than the integer n2.
n1 -le n2 True if the integer n1 is algebraically less than or equal to the integer n2.
If file is a symbolic link, test will fully dereference it and then evaluate the expression against the file referenced, except for the -h and -L primaries.
These primaries can be combined with the following operators:
! expression True if expression is false.
n1 -ge n2 True if the integer n1 is algebraically greater than or equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less than the integer n2.
n1 -le n2 True if the integer n1 is algebraically less than or equal to the integer n2.
If file is a symbolic link, test will fully dereference it and then evaluate the expression against the file referenced, except for the -h and -L primaries.
These primaries can be combined with the following operators:
! expression True if expression is false.
expression1 -a expression2
True if both expression1 and expression2 are true.
expression1 -o expression2
True if either expression1 or expression2 are true.
( expression )
True if expression is true.
The -a operator has higher precedence than the -o operator.
Some shells may provide a builtin test command which is similar or identical to this utility. Consult the builtin(1) manual page.
GRAMMAR AMBIGUITY
The test grammar is inherently ambiguous. In order to assure a degree of consistency, the cases described in the IEEE Std 1003.2 (``POSIX.2''), section D11.2/4.62.4, standard are evaluated consistently
according to the rules specified in the standards document. All other cases are subject to the ambiguity in the command semantics.
In particular, only expressions containing -a, -o, ( or ) can be ambiguous.
EXIT STATUS
The test utility exits with one of the following values:
0 expression evaluated to true.
1 expression evaluated to false or expression was missing.
>1 An error occurred.
EXAMPLES
Implement test FILE1 -nt FILE2 using only POSIX functionality:
test -n "$(find -L -- FILE1 -prune -newer FILE2 2>/dev/null)"
This can be modified using non-standard find(1) primaries like -newerca to compare other timestamps.
COMPATIBILITY
For compatibility with some other implementations, the = primary can be substituted with == with the same meaning.
SEE ALSO
builtin(1), expr(1), find(1), sh(1), stat(1), symlink(7)
STANDARDS
The test utility implements a superset of the IEEE Std 1003.2 (``POSIX.2'') specification. The primaries <, ==, >, -ef, -nt, -ot, -G, and -O are extensions.
BUGS
Both sides are always evaluated in -a and -o. For instance, the writable status of file will be tested by the following command even though the former expression indicated false, which results in a
gratuitous access to the file system:
[ -z abc -a -w file ]
To avoid this, write
[ -z abc ] && [ -w file ]
BSD June 1, 2013 BSD
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