`csh` Scripting

Last modified Friday, 20-Jan-2023 17:41:30 UTC.

Recall the Introduction to Shells, especially the Idea of Scripts
Variables
- Shell variables have names consisting of up to 20 letters and digits starting with a letter. The underscore character is considered a letter.
- Variables have no defined type, and may be heterogeneous.
- When used as an lvalue, a variable is given directly. When used as an rvalue, it is preceded by a $, and optionally surrounded by {}s to insulate it from following characters which would otherwise be part of it..
- Variables may be introduced in several contexts, most commonly with a set command, e.g.,
```
set MyData = 27 
```
  or an @ command, which is used for numeric variables, e.g.,
```
@ MyAnswer = $MyData + 16 
```
- List/array variables hold multiple values. Lists are written using ()s, and the elements are blank separated, e.g.,
```
set MyList = (the cat is 27 years old) 
```
  Members of list variables are accessed using []s, indexing from 1, e.g.,
```
echo $MyList[4]
set MyList[4] = 16
echo $MyList 
```
  Sublists are accesed by giving a range, using a -, e.g.,
```
echo $MyList[2-4] 
```
Predefined variables
- $?name - 1 if name is set, 0 if it is not.
- $0 - The name of the file from which command input is being read.
- $argv - The list of arguments to the shell (script)
- $number - Equivalent to $argv[number]
- $* - Equivalent to $argv, which is equivalent to $argv[*]
- $#name - The number of words in name.
- $# - Equivalent to $#argv
- $%name - The number of characters in name.
- $$ - The (decimal) process number of the shell.
- $? - The status from the last command.
- $< - A line from the standard input
- Predefined - Script full of predefined variables
Expressions
- The operators are similar to those of C and have the same precedence. They include
```
||  &&  |  ^  &  ==  !=  =~  !~  <=  >=
<  > <<  >>  +  -  *  /  %  !  ~  (  ) 
```
- ShellArithmetic - Script using arithmetic
- The ==, !=, =~, and !~ operators compare their arguments as strings; all others operate on numbers.
- The operators =~ and !~ are like != and == except that the right hand side is a glob-pattern against which the left hand operand is matched.
- The results of all expressions are strings, which represent decimal numbers.
- A command in ``s evaluates to the stdout output of the command.
- ShellStrings - Script using strings
- File inquiry operators
  - Expressions of the form -X filename evaluate to 1 or 0, for true and false.
  - Some value for for X are:
    - r - Read access
    - w - Write access
    - x - Execute access
    - X - Executable in the path or shell builtin
    - e - Existence
    - o - Ownership
    - z - Zero size
    - s - Non-zero size
    - f - Plain file
    - d - Directory
    - l - Symbolic link
- ShellFiles - Script using file properties
Builtin commands (a small selection!)
- @ name++|-- Increments or decrements name
- break - Causes execution to resume after the end of the nearest enclosing foreach or while.
- continue - Continues execution of the nearest enclosing while or foreach. The rest of the commands on the current line are executed.
- echo [-n] word ... - Writes each word to the shell's standard output, separated by spaces, and terminated with a newline unless -n is used.
- exit [expr] - The shell exits either with the value of the specified expr
- ```
foreach name (wordlist)
       ...
end
```
  Successively sets the variable name to each member of wordlist and executes the sequence of commands between this command and the matching end.
- ```
if (expr) then
       ...
else if (expr2) then
       ...
else
       ...
endif
```
  If the specified expr is true then the commands to the first else are executed; otherwise if expr2 is true then the commands to the second else are executed, etc. Only one endif is needed. The else part is optional.
- shift [variable] - Without variable, discards argv[1] and shifts the members of argv to the left. With variable, performs the same function on variable.
- source [-h] name [args ...] - The shell reads and executes commands from name.
- ```
switch (string)
case str1:
    ...
    breaksw
...
default:
    ...
    breaksw
endsw
```
  Each case label is successively matched against the specified string.
- wait - The shell waits for all background jobs.
- ```
while (expr)
       ...
end 
```
  Executes the commands between the while and the matching end while expr evaluates non-zero.
- Builtins - Scripts to exercise some builtins

csh Scripting

Exercises

`csh` Scripting