Expressions (Linux in a Nutshell, 3rd Edition)

Expressions are used in @, if, and while statements to perform arithmetic, string comparisons, file testing, and so on. exit and set also specify expressions, as can the tcsh built-in command filetest. Expressions are formed by combining variables and constants with operators that resemble those in the C programming language. Operator precedence is the same as in C but can be remembered as follows:

Group all other expressions inside parentheses. Parentheses are required if the expression contains <, >, &, or |.

8.5.1. Operators

Operators can be one of the following types:

8.5.1.1. Assignment operators

Operator	Description
`=`	Assign value.
`+= -=`	Reassign after addition/subtraction.
`*= /= %=`	Reassign after multiplication/division/remainder.
`&= ^= \|=`	Reassign after bitwise AND/XOR/OR.
`++`	Increment.
`--`	Decrement.

8.5.1.2. Arithmetic operators

Operator	Description
`* / %`	Multiplication; integer division; modulus (remainder)
`+ -`	Addition; subtraction

8.5.1.3. Bitwise and logical operators

Operator	Description
`~`	Binary inversion (one's complement).
`!`	Logical negation.
`<< >>`	Bitwise left shift; bitwise right shift.
`&`	Bitwise AND.
`^`	Bitwise exclusive OR.
`\|`	Bitwise OR.
`&&`	Logical AND.
`\|\|`	Logical OR.
`{` command `}`	Return 1 if command is successful, 0 otherwise. Note that this is the opposite of command's normal return code. The $status variable may be more practical.

8.5.1.4. Comparison operators

Operator	Description
`== !=`	Equality; inequality
`<= >=`	Less than or equal to; greater than or equal to
`< >`	Less than; greater than

8.5.1.5. File inquiry operators

Command substitution and filename expansion are performed on file before the test is performed. tcsh permits operators to be combined (e.g., -ef). The following is a list of the valid file inquiry operators:

Operator	Description
`-d` file	The file is a directory.
`-e` file	The file exists.
`-f` file	The file is a plain file.
`-o` file	The user owns the file.
`-r` file	The user has read permission.
`-w` file	The user has write permission.
`-x` file	The user has execute permission.
`-z` file	The file has 0 size.
`!`	Reverse the sense of any preceding inquiry.

Some additional operators specific to tcsh are:

Operator	Description
`-b` file	The file is a block special file.
`-c` file	The file is a character special file.
`-g` file	The file's set-group-ID bit is set.
`-k` file	The file's sticky bit is set.
`-l` file	The file is a symbolic link.
`-L` file	Apply any remaining operators to symbolic link, not the file it points to.
`-p` file	The file is a named pipe (FIFO).
`-s` file	The file has nonzero size.
`-S` file	The file is a socket special file.
`-t` file	file is a digit and is an open file descriptor for a terminal device.
`-u` file	The file's set-user-ID bit is set.
`-X` file	The file is executable and is in the path or is a shell built-in.

Finally, tcsh provides the following operators, which return other kinds of information:

Operator	Description
`-A[:]` file	Last time file was accessed, as the number of seconds since the Epoch. With a colon (:), the result is in timestamp format.
`-C[:]` file	Last time inode was modified. With a colon (:), the result is in timestamp format.
`-D` file	Device number.
`-F` file	Composite file identifier, in the form device:inode.
`-G[:]` file	Numeric group ID for the file. With a colon (:), the result is the group name if known; otherwise, the numeric group ID.
`-I` file	Inode number.
`-L` file	The name of the file pointed to by symbolic link file.
`-M[:]` file	Last time file was modified. With a colon (:), the result is in timestamp format.
`-N` file	Number of hard links.
`-P[:]` file	Permissions in octal, without leading 0. With a colon (:), the result includes a leading 0.
`-P`mode`[:]` file	Equivalent to `-P` file ANDed to mode. With a colon (:), the result includes a leading 0.
`-U[:]` file	Numeric user ID of the file's owner. With a colon (:), the result is the username if known, otherwise the numeric user ID.
`-Z` file	The file's size, in bytes.

Expression	Value of $x
`@ x = ($n > 10 \|\| $n < 5)`	1
`@ x = ($n >= 0 && $n < 3)`	0
`@ x = ($n << 2)`	16
`@ x = ($n >> 2)`	1
`@ x = $n % 2`	0
`@ x = $n % 3`	1

Expression	Meaning
`while ($#argv != 0)`	While there are arguments . . .
`if ($today[1] == <">Fri<">)`	If the first word is "Fri". . .
`if (-f $argv[1])`	If the first argument is a plain file. . .
`if (! -d $tmpdir)`	If `tmpdir` is not a directory. . .