Chapter 9: The TC Shell
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Transcript of 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