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Chapter 9: The TC Shell. Everything you thought you knew is now wrong. In this chapter …. Background Accessing tcsh Startup Files Commonality with bash Standard error Variables Control structures and builtins. Background. TC Shell - an expansion of the C Shell from BSD

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chapter 9 the tc shell

Chapter 9:The TC Shell

Everything you thought you knew is now wrong

in this chapter
In this chapter …
  • Background
  • Accessing tcsh
  • Startup Files
  • Commonality with bash
  • Standard error
  • Variables
  • Control structures and builtins
background
Background
  • TC Shell - an expansion of the C Shell from BSD
  • The ‘T’ comes from TENEX and TOPS-20 OSes
  • Meant to add features such as command completion to csh
  • Not a very good scripting language – bash is much better
  • Most features in bash are in tcsh, just might have different syntax
shell scripting caveat
Shell Scripting Caveat
  • Recall that we can specify which shell to use for a script by starting the first line with #!/path_to_shell
  • Without this line, tcsh will use sh to execute the script unless you run the script with tcsh explicitly
  • Different than bash and other shells
  • Even tcsh admits it’s not great
accessing tcsh
Accessing tcsh
  • Easiest way is to just issue tcsh
  • Want to change your login shell to tcsh?
    • chsh
  • Exiting tcsh
    • exit
    • logout (only if login shell)
    • CTRL-D (if ignoreeof not set)
startup files
Startup Files
  • Logon shell
    • /etc/csh.cshrc
    • /etc/csh.logon
    • ~/.tcshrc or ~/.cshrc
    • ~/.login
  • Non-logon shell
    • /etc/csh.cshrc
    • ~/.tcshrc or ~/.cshrc
more files
More files
  • On logout
    • /etc/csh.logout
    • ~/.logout
  • History events loaded from
    • ~/.history
things in common with bash
Things in common with bash
  • Command line expansion
    • Called substitution in tcsh docs
  • History
    • history builtin works the same
    • ! history references work the same
    • Instead of HISTSIZE and HISTFILESIZE, tcsh uses history and savehist
    • If variable histlit is set, history shows literal commands before command line substitution
tcsh vs bash con t
tcsh vs. bash con’t
  • Aliases
    • Syntax: alias name “value”
    • Allows you to reference command line arguments using \!* or \!:n
    • Special aliases
      • beepcmd – instead of bell
      • cwdcmd – whenever you change directories
      • periodic – a periodic command to run (tperiod)
      • precmd – runs just before shell prompt
      • shell – absolute path to use for scripts w/o #!
tcsh vs bash con t1
tcsh vs. bash con’t
  • Job control
    • Almost identical, slightly different for multiple processes spawned at once
  • Filename substitution
    • *, ?, [], {}, ~ all the same
    • ~+,~- not available
  • Directory stack – same
  • Command Substitution $() – NOT available
    • Use `command` instead
standard error
Standard Error
  • tcsh doesn’t have an easy way to capture standard error like bash (i.e. 2> )
  • Instead we have to use >& to combine standard error and standard out
  • So grab standard out first, then combine out and error to capture error
  • Ex:

(cat x y > results) >& errors

word completion
Word Completion
  • Tab completion is similar in tcsh
  • Start with an ambiguous reference then hit tab
  • If there are multiple matches, it will maximize the length of the prefix
  • Will *not* show a list of possible matches unless you press CTRL-D
variables
Variables
  • In tcsh, there are two scopes, local and global
  • To declare a local variable, use:

set variable = value

    • Note the spaces – different than bash
  • To declare a local integer variable:

@ variable = value

  • To declare a global (avail. to child procs):

setenv variable value

variables con t
Variables con’t
  • Just like bash, use a $ to reference a variable’s contents (also ${ } )
  • unset, unsetenv removes variables
  • To declare an array of strings:

set variable = ( values … )

  • To reference single entries use [] operator (base 1)

echo variable[2]

set variable[4] = “Value”

numeric variables
Numeric Variables
  • The @ builtin lets you work with numeric expressions
  • Ex.

@ variable = ( $count + 4 ) / 5

  • Caveat – separate each element by a space
  • Available operators:

=, +=, -=, *=, /=, %=, +, -, *, /, %

numeric variables con t
Numeric Variables con’t
  • To make an array of numeric variables, you actually have to use set

set variable = (1 2 3 4 5)

  • Then use @ to access the array

@ variable[2] = 2

more variable goodness
More variable goodness
  • $#variable – displays the number of elements in a variable array
  • $?variable – displays whether the variable is set (1 for set, 0 for not set)
  • To read user input into a variable, set the variable to “$<“
    • E.g. in a script
      • echo –n “Enter a value:”set usr_input = “$<“
important shell variables
Important Shell Variables
  • autologout – set timeout period
  • cwd – contains current working directory
  • histfile – contains location of history
  • history – how many history items to keep
  • home – your home directory
  • mail – where your mail is stored
  • owd – previous (old) working directory
shell variables con t
Shell Variables, con’t
  • histlit – show literal commands in history
  • ignoreeof – ignore CTRL-D
  • nobeep – disables shell beeps
  • noglob – turns of file globbing
  • rmstar – prompt for rm * commands
  • visiblebell – causes screen to flash for bells
shell variables con t1
Shell Variables, con’t
  • argv – array of command line args
    • $n , $*
    • Use $#argv to get number of args
slide21
Misc
  • bindkey
  • Control structures
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