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 set set 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

Chapter 9: The TC Shell