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10. tc Shell (Part 1)

tc Shell






Introduction

  • The tcsh is a new version of the C shell, which originated from Unix BSD
  • The C shell and tcsh base much of their syntax on the C language
  • From the beginning, the Bourne shell and the C shell have had different features and syntax
  • However bash and tcsh now share many of the same features and their syntax are more similar, but there is a bigger difference between bash and tcsh than bash and ksh
  • In this note, we discuss the differences between tcsh and bash

Running the TC Shell

  • tcsh can be the default login shell, just like bash or ksh
  • The default tcsh prompt (stored in a variable called prompt) is:  $
  • From any shell, to start tcsh:  tcsh
  • To check where tcsh is in the system:  whereis  tcsh
    • Normally tcsh is at:  /bin/tcsh
  • If tcsh is started as a child process of another shell, to get back to the original shell, use:  exit
  • To find the current shell that is running:  echo $0

Input / Output

  • To print to screen, use echo (just as with bash)
  • To read from the input stream, use set and $<
  • An example script:  
#!/bin/tcsh
echo -n "Your name: "
set name = "$<" # read input line into name
echo “hello $name” # print name
  • Just like with any other shell, tcsh script also accepts command line arguments and options
  • Command line arguments are accessed through the positional parameters(see discussion of positional variables in slide 7)
  • We can also use redirection of both input and output of the shell
  • Redirection of error is not possible without also redirection of output A common way to redirect error is:  
( command  >  outputfile )  >& errfile
  • We can also use command substitutions and pipes for input and output. Command substitutions typically use the ` ` (back ticks) format.

User Defined Variables

  • There are 2 ways to create and initialize variables, depending on whether the data is a string or a number
  • Create and initialize string variables with set
  • Format:   set  varName = value      
    • where space before and after the = is okay
    • For example:   set  class = “cis18c”
  • Due to the fickle nature of the tcsh parser, always quote strings and string variables, especially if the string has space and punctuation characters
  • Create and initialize numeric variables with @
  • Format:   @ varName = value      
       where space before and after the = is okay
and there is a space between @ and the variable name
For example:   @  num = 10
  • $?varName  returns 1 if variable is set, 0 if variable is undefined

Arrays

  • Create and initialize arrays with set and ( )
  • Format:   set  arrayName  =  (val1  val2  val3)
  • Array elements are accessed by using [ ] but indexing starts at 1, not 0
  • Multiple index values or * can be put in between [ ] to access multiple elements at a time
  • Example:   set   colors = (red  orange  yellow  green  blue)
  echo  $colors[1]     print:  red
  echo  $colors[2-4] print:  orange yellow green
  echo  $colors[*] print all elements
  • Note: there is no space when using index values inside [ ]
echo  $colors[1]  is okay, but echo  $colors[ 1 ]  is not okay
  • $arrayName refers to the entire array
  • $#arrayName  returns the number of elements in an array
  • shift  arrayName  shifts left the elements of the array and the array is shorter by one

Shell Variables

  • Command line arguments are in positional variables ($0, $1, $2, $*) but there is no $@ 
  • There is no getopts function that can be used for option validation
  • Following the tradition of the C language, command line options and arguments are also stored in the argv array, and this is the common way to access them in a tcsh script
$argv[1]   first input argument
$argv[2] second input argument
$argv[n] nth input argument
$argv entire list of arguments
$#argv number of input arguments
  • Process ID is stored in the variable $, just as in bash
  • Exit status is stored in the variable status or ?

Environment Variables

  • System environment variables:
  • Some environment variables can be in lowercase
  • Some common environment variables:
prompt               primary shell prompt
home     or HOME      home directory
path       or PATH     search path
user       or  USER             user id
shell                           current shell
0             current shell

Expressions

  • Arithmetic expressions
    • Operators are similar to bash:  +, -, *, /, +=, /= , ++…
    • For the shell to evaluate the expression, the expression must begin with @
    • For example:  @ num = 2 * $var    num is 2 times var
or     @ count++ count is incremented by 1
  • Logical expressions
    • Operators are similar to bash:   !,  &&,  ||
  • Relational expressions
    • Operators are similar to bash:  <,  >,  <=,  >=,  !=,  ==
    • These logical operators apply to string comparison as well as numeric comparison.  Depending on the data in the variables, the operation can be a string compare or a numeric compare
    • When using == or != with strings, the operator requires an exact match

String matching 

  • Strings can also be partially matched with the operators   
    • =~   (for match)   and    !~    (for not match)
  • The right side of =~ and !~ are wildcards, not regex
  • In other words, the match works the same way as how the shell matches wildcards on the command line
  • Example:     “$str” =~ “cis*”
    • will match if $str starts with cis, followed by 0 or more characters
  • File tests are similar to bash:
-e exist -r    read access
-z zero length -w   write access
-f regular file -x    execute access
-d directory
-l link


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