10. tc Shell (Part 1)

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