Chapter 9: The TC Shell

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• 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• 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• 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• Logon shell

– /etc/csh.cshrc– /etc/csh.logon– ~/.tcshrc or ~/.cshrc– ~/.login

• Non-logon shell– /etc/csh.cshrc– ~/.tcshrc or ~/.cshrc

More files• On logout

– /etc/csh.logout– ~/.logout

• History events loaded from– ~/.history

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• 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’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• 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• 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• 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• 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• 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• To make an array of numeric variables, you

actually have to use setset variable = (1 2 3 4 5)

• Then use @ to access the array@ variable[2] = 2

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• 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• 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’t• argv – array of command line args

– $n , $*– Use $#argv to get number of args

Misc• bindkey• Control structures