GNU Bash Reference Manual by Chet Ramey & Brian Fox

Author:Chet Ramey & Brian Fox
Language: eng
Format: mobi
Publisher: UNKNOWN
Published: 2018-06-16T23:00:00+00:00

6.4 Bash Conditional Expressions

Conditional expressions are used by the [[ compound command and the test and [ builtin commands.

Expressions may be unary or binary. 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 and file2 refer to the same device and inode numbers.

file1 -nt file2

True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

file1 -ot file2

True if file1 is older than file2, or if file2 exists and file1 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 the set builtin (see Section 4.3.1 [The Set Builtin], page 60).

-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.

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