linux 命令 之判断表达式

摘自http://www.comptechdoc.org/os/linux/usersguide/linux_ugshellpro.html

Tests

There is a function provided by bash called test which returns a true or false value depending on the result of the tested expression. Its syntax is:

test expression

It can also be implied as follows:

[ expression ]

The tests below are test conditions provided by the shell:

• -b file = True if the file exists and is block special file.
• -c file = True if the file exists and is character special file.
• -d file = True if the file exists and is a directory.
• -e file = True if the file exists.
• -f file = True if the file exists and is a regular file
• -g file = True if the file exists and the set-group-id bit is set.
• -k file = True if the files‘ "sticky" bit is set.
• -L file = True if the file exists and is a symbolic link.
• -p file = True if the file exists and is a named pipe.
• -r file = True if the file exists and is readable.
• -s file = True if the file exists and its size is greater than zero.
• -s file = True if the file exists and is a socket.
• -t fd = True if the file descriptor is opened on a terminal.
• -u file = True if the file exists and its set-user-id bit is set.
• -w file = True if the file exists and is writable.
• -x file = True if the file exists and is executable.
• -O file = True if the file exists and is owned by the effective user id.
• -G file = True if the file exists and is owned by the effective group id.
• file1 –nt file2 = True if file1 is newer, by modification date, than file2.
• file1 ot file2 = True if file1 is older than file2.
• file1 ef file2 = True if file1 and file2 have the same device and inode numbers.
• -z string = True if the length of the string is 0.
• -n string = True if the length of the string is non-zero.
• string1 = string2 = True if the strings are equal.
• string1 != string2 = True if the strings are not equal.
• !expr = True if the expr evaluates to false.
• expr1 –a expr2 = True if both expr1 and expr2 are true.
• expr1 –o expr2 = True is either expr1 or expr2 is true.