Editing Thompson shell (section)

== Design ==
The shell's design was intentionally minimalistic; even the ''if'' and ''goto'' statements, essential for control of program flow, were implemented as separate commands.<ref name=Mashey1976-10-13/>

The shell has no facilities for comments besides a builtin command {{code|:}}. Programmers simply write text after this command, which ignores all parameters and simply succeeds. Other builtins include {{code|chdir}}, {{code|exit}}, {{code|login}}, {{code|newgrp}}, {{code|shift}}, and {{code|wait}}.<ref name=etsh.man/>

The ''if'' command combines the uses of modern-day Bourne shell ''test'' and ''if''. The command first looks for an expression (which can be similar to modern-day ''test'' or involve an external command) then treats the rest of the command-line as the command to execute if the condition turns out true. There is no ''else'' branch.<ref>{{cite web |title=if(1) (html) - IF(1) - Manuals - Etsh Project (V6Shell) |url=https://etsh.nl/man/_if.1.html |website=etsh.nl}}</ref>

''goto'' is implemented in an interesting way, as it is separate from the shell. When asked to jump to "LABEL", it seeks the current command file for a line that says {{code|: LABEL}} (recall that {{code|:}} is simply ignored), then exits. When the shell tries to read a next line, the repositioned file descriptor will direct it to the labelled location.<ref>{{cite web |title=goto(1) (html) - GOTO(1) - Manuals - Etsh Project (V6Shell) |url=https://etsh.nl/man/_goto.1.html |website=etsh.nl}}</ref>

There is no redirection of additional file descriptors other than standard input and output (0 and 1) in Thompson shell. Redirection of stderr (file descriptor 2) also requires an external program wrapper, {{code|fd2}}.<ref>{{cite web |title=fd2(1) (html) - FD2(1) - Manuals - Etsh Project (V6Shell) |url=https://etsh.nl/man/_fd2.1.html |website=etsh.nl}}</ref>

The shell supports globbing,<ref name=etsh.man>{{cite web |title=tsh(1) (html) - TSH(1) - Manuals - Etsh Project (V6Shell) |url=https://etsh.nl/man/_tsh.1.html |website=etsh.nl}}</ref> but actually implements it by deferring it to a {{code|glob}} command that replaces the arguments and calls the requested command.<ref>{{cite web |title=glob(1) (html) - GLOB(1) - Manuals - Etsh Project (V6Shell) |url=https://etsh.nl/man/_glob.1.html |website=etsh.nl}}</ref>

Thompson shell has positional parameters, but no named variables nor access to environmental variables. It understands the creation of background commands with {{code|&}}, similar to Bourne shell. It offers quoting and backslash escapes, though the single quotes work differently from Bourne shell.<ref name=etsh.man/>