Bash/Bash-Conditional-Expressions
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6.4 Bash Conditional Expressions
Conditional expressions are used by the [[
compound command
and the test
and [
builtin commands. The test
and [
commands determine their behavior based on the number
of arguments; see the descriptions of those commands for any other
command-specific actions.
Expressions may be unary or binary,
and are formed from the following primaries.
Unary expressions are often used to examine the status of a file.
There are string operators and numeric comparison operators as well.
Bash handles several filenames specially when they are used in
expressions.
If the operating system on which Bash is running provides these
special files, Bash will use them; otherwise it will emulate them
internally with this behavior:
If the file
argument to one of the primaries is of the form
/dev/fd/N
, then file descriptor N
is checked.
If the file
argument to one of the primaries is one of
/dev/stdin
, /dev/stdout
, or /dev/stderr
, file
descriptor 0, 1, or 2, respectively, is checked.
When used with [[
, the ‘<
’ and ‘>
’ operators sort
lexicographically using the current locale.
The test
command uses ASCII ordering.
Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.
-a file
True if
file
exists.-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.-f file
True if
file
exists and is a regular file.-g file
True if
file
exists and its set-group-id bit is set.-h file
True if
file
exists and is a symbolic link.-k file
True if
file
exists and its "sticky" bit is set.-p file
True if
file
exists and 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 fd
True if file descriptor
fd
is open and refers to a terminal.-u file
True if
file
exists and its set-user-id bit is set.-w file
True if
file
exists and is writable.-x file
True if
file
exists and is executable.-G file
True if
file
exists and is owned by the effective group id.-L file
True if
file
exists and is a symbolic link.-N file
True if
file
exists and has been modified since it was last read.-O file
True if
file
exists and is owned by the effective user id.-S file
True if
file
exists and is a socket.file1 -ef file2
True if
file1
andfile2
refer to the same device and inode numbers.file1 -nt file2
True if
file1
is newer (according to modification date) thanfile2
, or iffile1
exists andfile2
does not.file1 -ot file2
True if
file1
is older thanfile2
, or iffile2
exists andfile1
does not.-o optname
True if the shell option
optname
is enabled. The list of options appears in the description of the-o
option to theset
builtin (see The Set Builtin).-v varname
True if the shell variable
varname
is set (has been assigned a value).-R varname
True if the shell variable
varname
is set and is a name reference.-z string
True if the length of
string
is zero.-n string
string
True if the length of
string
is non-zero.string1 == string2
string1 = string2
True if the strings are equal. When used with the
[[../
command, this performs pattern matching as described above (see Conditional Constructs).‘
=
’ should be used with thetest
command for POSIX conformance.string1 != string2
True if the strings are not equal.
string1 < string2
True if
string1
sorts beforestring2
lexicographically.string1 > string2
True if
string1
sorts afterstring2
lexicographically.arg1 OP arg2
OP
is one of ‘-eq
’, ‘-ne
’, ‘-lt
’, ‘-le
’, ‘-gt
’, or ‘-ge
’. These arithmetic binary operators return true ifarg1
is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal toarg2
, respectively.Arg1
andarg2
may be positive or negative integers. When used with the[[../
command,Arg1
andArg2
are evaluated as arithmetic expressions (see Shell Arithmetic).
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