<s>
tcsh	B-Language
(	O
“	O
tee-see-shell	O
”	O
,	O
“	O
tee-shell	O
”	O
,	O
or	O
as	O
“	O
tee	O
see	O
ess	O
aitch	O
”	O
,	O
tcsh	B-Language
)	O
is	O
a	O
Unix	B-Application
shell	I-Application
based	O
on	O
and	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
the	O
C	B-Operating_System
shell	I-Operating_System
(	O
csh	O
)	O
.	O
</s>
<s>
It	O
is	O
essentially	O
the	O
C	B-Operating_System
shell	I-Operating_System
with	O
programmable	O
command-line	B-Application
completion	O
,	O
command-line	B-Application
editing	O
,	O
and	O
a	O
few	O
other	O
features	O
.	O
</s>
<s>
Unlike	O
the	O
other	O
common	O
shells	O
,	O
functions	O
cannot	O
be	O
defined	O
in	O
a	O
tcsh	B-Language
script	O
and	O
the	O
user	O
must	O
use	O
aliases	O
instead	O
(	O
as	O
in	O
csh	O
)	O
.	O
</s>
<s>
It	O
is	O
the	O
native	O
root	O
shell	O
for	O
BSD-based	O
systems	O
such	O
as	O
FreeBSD	B-Operating_System
.	O
</s>
<s>
tcsh	B-Language
added	O
filename	O
and	O
command	O
completion	O
and	O
command	B-Application
line	I-Application
editing	O
concepts	O
borrowed	O
from	O
the	O
TENEX	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
is	O
the	O
source	O
of	O
the	O
“	O
t	O
”	O
.	O
</s>
<s>
Because	O
it	O
only	O
added	O
functionality	O
and	O
did	O
not	O
change	O
what	O
was	O
there	O
,	O
tcsh	B-Language
remained	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
the	O
original	O
C	B-Operating_System
shell	I-Operating_System
.	O
</s>
<s>
Though	O
it	O
started	O
as	O
a	O
side	O
branch	O
from	O
the	O
original	O
csh	O
source	O
tree	O
that	O
Bill	O
Joy	O
had	O
created	O
,	O
tcsh	B-Language
is	O
now	O
the	O
main	O
branch	O
for	O
ongoing	O
development	O
.	O
</s>
<s>
tcsh	B-Language
is	O
very	O
stable	O
but	O
new	O
releases	O
continue	O
to	O
appear	O
roughly	O
once	O
a	O
year	O
,	O
consisting	O
mostly	O
of	O
minor	O
bug	O
fixes	O
.	O
</s>
<s>
On	O
many	O
systems	O
,	O
such	O
as	O
macOS	B-Application
and	O
Red	O
Hat	O
Linux	O
,	O
csh	O
is	O
actually	O
tcsh	B-Language
.	O
</s>
<s>
Often	O
one	O
of	O
the	O
two	O
files	O
is	O
either	O
a	O
hard	B-Application
link	I-Application
or	O
a	O
symbolic	B-Application
link	I-Application
to	O
the	O
other	O
,	O
so	O
that	O
either	O
name	O
refers	O
to	O
the	O
same	O
improved	O
version	O
of	O
the	O
C	B-Operating_System
shell	I-Operating_System
.	O
</s>
<s>
On	O
Debian	O
and	O
some	O
derivatives	O
(	O
including	O
Ubuntu	B-Operating_System
)	O
,	O
there	O
are	O
two	O
different	O
packages	O
:	O
csh	O
and	O
tcsh	B-Language
.	O
</s>
<s>
The	O
former	O
is	O
based	O
on	O
the	O
original	O
BSD	O
version	O
of	O
csh	O
and	O
the	O
latter	O
is	O
the	O
improved	O
tcsh	B-Language
.	O
</s>
<s>
The	O
“	O
t	O
”	O
in	O
tcsh	B-Language
comes	O
from	O
the	O
“	O
T	O
”	O
in	O
TENEX	B-Operating_System
,	O
an	O
operating	B-General_Concept
system	I-General_Concept
which	O
inspired	O
Ken	O
Greer	O
at	O
Carnegie	O
Mellon	O
University	O
,	O
the	O
author	O
of	O
tcsh	B-Language
,	O
with	O
its	O
command-completion	O
feature	O
.	O
</s>
<s>
Greer	O
began	O
working	O
on	O
his	O
code	O
to	O
implement	O
Tenex-style	O
file	O
name	O
completion	O
in	O
September	O
1975	O
,	O
finally	O
merging	O
it	O
into	O
the	O
C	B-Operating_System
shell	I-Operating_System
in	O
December	O
1981	O
.	O
</s>
<s>
Tcsh	B-Language
is	O
the	O
only	O
shell	O
that	O
provides	O
this	O
feature	O
.	O
</s>
<s>
Early	O
versions	O
of	O
Mac	B-Application
OS	I-Application
X	O
shipped	O
with	O
tcsh	B-Language
as	O
the	O
default	O
shell	O
,	O
but	O
the	O
default	O
for	O
new	O
accounts	O
became	O
bash	B-Operating_System
as	O
of	O
10.3	B-Operating_System
then	O
zsh	B-Application
as	O
of	O
10.15	B-Device
.	O
</s>
<s>
(	O
tcsh	B-Language
is	O
still	O
provided	O
,	O
and	O
upgrading	O
the	O
OS	O
does	O
not	O
change	O
the	O
shell	O
of	O
any	O
existing	O
accounts	O
)	O
.	O
</s>
<s>
tcsh	B-Language
is	O
the	O
default	O
root	O
shell	O
of	O
FreeBSD	B-Operating_System
(	O
the	O
default	O
user	O
shell	O
is	O
POSIX-based	O
)	O
and	O
its	O
descendants	O
like	O
DragonFly	B-Application
BSD	I-Application
and	O
DesktopBSD	B-Application
.	O
</s>
