[C shell] Variable operators and Expressions

cited from here

1. Variables

${ var }

The value of variable var .

${ var [ i ]}

Select word or words in position i of var . i can be a single number, a range m - n , a range - n (missing m implies 1), a range m - (missing n implies all remaining words), or * (select all words). i can also be a variable that expands to one of these values.

${# var }

The number of words in var .

${#argv}

The number of command-line arguments.

${argv[ n ]}

Individual arguments on command line (positional parameters). n is a number (1, 12, etc.).

${ n }

Same as ${argv[ n ]} .

${argv[*]}

All arguments on command line.

$*

Same as $argv[*] .

${argv[$#argv]}

The last argument.

${? var }

Return 1 if var is set; 0 if var is not set.

! ${? var }

Return 0 if var is set; 1 if var is not set.

$$

Process number of current shell; useful as part of a filename for creating temporary files with unique names.

$<

Read a line from standard input.

 

2. Expressions

  1. * / %

  2. + -

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

3. Operators

3.1 Assignment Operators

=

Assign value.

+= -=

Reassign after addition/subtraction.

*= /= %=

Reassign after multiplication/division/remainder.

&= ^= |=

Reassign after bitwise AND/XOR/OR.

++

Increment.

Decrement.

3.2 Arithmetic Operators

* / %

Multiplication; integer division; modulus (remainder).

c+ –

Addition; subtraction.

3.3 Bitwise and Logical Operators

~

Binary inversion (one’s complement).

!

Logical negation.

<< >>

Bitwise left shift; bitwise right shift.

&

Bitwise AND.

^

Bitwise exclusive OR.

|

Bitwise OR.

&&

Logical AND.

||

Logical OR.

{ cmd }

Return 1 if command cmd is successful; 0 otherwise. Note that this is the opposite of cmd ‘s normal return code. The status variable may be more practical.

3.4 Comparison Operators

== !=

Equality; inequality.

<= >=

Less than or equal to; greater than or equal to.

< >

Less than; greater than.

=~

String on left matches a filename pattern on the right containing * , ? , or [ ] .

!~

String on left does not match a filename pattern containing * , ? , or [ ] .

3.5 File Inquiry Operators

Command substitution and filename expansion are performed on file before the test is performed.

-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 zero size.

!

Reverse the sense of any of the above inquiries.

Examples.

The following examples show @ commands and assume n = 4:

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

The following examples show the first line of if or while statements:

Expression

Meaning

while ($#argv != 0)

While there are command-line ( argv ) arguments …

if ($today[1] == Fri)

If the first word is Fri

if ($file !~ *.[zZ])

If the file doesn’t end with .z or .Z

if ($argv[1] =~ chap?)

If the first argument is chap followed by a single character…

if (-f $argv[1])

If the first argument is a plain file…

if (! -d $tmpdir)

If tmpdir is not a directory…

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