|
|
|
OS X syntax : Logical, arithmetical and comparison operators
These operators are similar to those of C and have the same precedence.
They include
|| && | ^ & == != =~ !~ <= >=
< > << >> + - * / % ! ~ ( )
Here the precedence increases to the right, `==' `!=' `=~' and `!~',
`<=' `>=' `<' and `>', `<<' and `>>', `+' and `-', `*' `/' and `%'
being, in groups, at the same level. The `==' `!=' `=~' and `!~' oper-
ators compare their arguments as strings; all others operate on num-
bers. The operators `=~' and `!~' are like `!=' and `==' except that
the right hand side is a glob-pattern against which the left hand
operand is matched. This reduces the need for use of the switch builtin
command in shell scripts when all that is really needed is pattern
matching.
Strings which begin with `0' are considered octal numbers. Null or
missing arguments are considered `0'. The results of all expressions
are strings, which represent decimal numbers. It is important to note
that no two components of an expression can appear in the same word;
except when adjacent to components of expressions which are syntacti-
cally significant to the parser (`&' `|' `<' `>' `(' `)') they should
be surrounded by spaces.
Command exit status
Commands can be executed in expressions and their exit status returned
by enclosing them in braces (`{}'). Remember that the braces should be
separated from the words of the command by spaces. Command executions
succeed, returning true, i.e., `1', if the command exits with status 0,
otherwise they fail, returning false, i.e., `0'. If more detailed sta-
tus information is required then the command should be executed outside
of an expression and the status shell variable examined.
File inquiry operators
Some of these operators perform true/false tests on files and related
objects. They are of the form -op file, where op is one of
r Read access
w Write access
x Execute access
X Executable in the path or shell builtin, e.g., `-X ls' and `-X
ls-F' are generally true, but `-X /bin/ls' is not (+)
e Existence
o Ownership
z Zero size
s Non-zero size (+)
f Plain file
d Directory
l Symbolic link (+) *
b Block special file (+)
c Character special file (+)
p Named pipe (fifo) (+) *
S Socket special file (+) *
u Set-user-ID bit is set (+)
g Set-group-ID bit is set (+)
k Sticky bit is set (+)
t file (which must be a digit) is an open file descriptor for a
terminal device (+)
R Has been migrated (convex only) (+)
L Applies subsequent operators in a multiple-operator test to a
symbolic link rather than to the file to which the link points
(+) *
file is command and filename expanded and then tested to see if it has
the specified relationship to the real user. If file does not exist or
is inaccessible or, for the operators indicated by `*', if the speci-
fied file type does not exist on the current system, then all enquiries
return false, i.e., `0'.
These operators may be combined for conciseness: `-xy file' is equiva-
lent to `-x file && -y file'. (+) For example, `-fx' is true (returns
`1') for plain executable files, but not for directories.
L may be used in a multiple-operator test to apply subsequent operators
to a symbolic link rather than to the file to which the link points.
For example, `-lLo' is true for links owned by the invoking user. Lr,
Lw and Lx are always true for links and false for non-links. L has a
different meaning when it is the last operator in a multiple-operator
test; see below.
It is possible but not useful, and sometimes misleading, to combine
operators which expect file to be a file with operators which do not,
(e.g., X and t). Following L with a non-file operator can lead to par-
ticularly strange results.
Other operators return other information, i.e., not just `0' or `1'.
(+) They have the same format as before; op may be one of
A Last file access time, as the number of seconds since the
epoch
A: Like A, but in timestamp format, e.g., `Fri May 14 16:36:10
1993'
M Last file modification time
M: Like M, but in timestamp format
C Last inode modification time
C: Like C, but in timestamp format
D Device number
I Inode number
F Composite file identifier, in the form device:inode
L The name of the file pointed to by a symbolic link
N Number of (hard) links
P Permissions, in octal, without leading zero
P: Like P, with leading zero
Pmode Equivalent to `-P file & mode', e.g., `-P22 file' returns
`22' if file is writable by group and other, `20' if by
group only, and `0' if by neither
Pmode: Like Pmode:, with leading zero
U Numeric userid
U: Username, or the numeric userid if the username is unknown
G Numeric groupid
G: Groupname, or the numeric groupid if the groupname is
unknown
Z Size, in bytes
Only one of these operators may appear in a multiple-operator test, and
it must be the last. Note that L has a different meaning at the end of
and elsewhere in a multiple-operator test. Because `0' is a valid
return value for many of these operators, they do not return `0' when
they fail: most return `-1', and F returns `:'.
If the shell is compiled with POSIX defined (see the version shell
variable), the result of a file inquiry is based on the permission bits
of the file and not on the result of the access(2) system call. For
example, if one tests a file with -w whose permissions would ordinarily
allow writing but which is on a file system mounted read-only, the test
will succeed in a POSIX shell but fail in a non-POSIX shell.
File inquiry operators can also be evaluated with the filetest builtin
command
Related commands:
OS X Syntax
