Editing C shell (section)

=== More like C ===

The Unix system had been written almost exclusively in C, so the C shell's first objective was a command language that was more stylistically consistent with the rest of the system.  The keywords, the use of parentheses, and the C shell's built-in expression grammar and support for arrays were all strongly influenced by C.

By today's standards, C shell may not seem particularly more C-like than many other popular scripting languages.  But through the 1980s and '90s, the difference was seen as striking, particularly when compared to [[Bourne shell]] (also known as ''sh''), the then-dominant shell written by [[Stephen R. Bourne|Stephen Bourne]] at [[Bell Labs]].  This example illustrates the C shell's more conventional [[Operator (computer programming)|expression operators]] and [[Syntax (programming languages)|syntax]].

{{column
|width=60em
|1=
Bourne shell
<syntaxhighlight lang="bash">
#!/bin/sh
if [ $days -gt 365 ]
then
   echo This is over a year.
fi
</syntaxhighlight>
|2=
C shell
<syntaxhighlight lang="csh">
#!/bin/csh
if ( $days > 365 ) then
   echo This is over a year.
endif
</syntaxhighlight>
}}

The Bourne sh lacked an [[Expression (programming)|expression grammar]]. The square bracketed condition had to be evaluated by the slower means of running the external [[test (Unix)|test]] program.  sh's <code>if</code> command took its argument words as a new command to be run as a [[Process (computing)|child process]].  If the child exited with a zero [[return code]], sh would look for a <code>then</code> clause (a separate statement, but often written joined on the same line with a semicolon) and run that nested block.  Otherwise, it would run the else. [[Hard link|Hard-linking]] the test program as both "<code>test</code>" and "<code>[</code>" gave the notational advantage of the square brackets and the appearance that the functionality of test was part of the sh language.  sh's use of a reversed keyword to mark the end of a control block was a style borrowed from [[ALGOL 68]].<ref>[https://groups.google.com/group/comp.lang.misc/msg/d58db4799c33e093?hl=en&dmode=source ''Re: Late Bloomers Revisited''] USENET post to comp.lang.misc by Piercarlo "Peter" Grandi, Dept of CS, UCW Aberystwyth, UK, 17 December 1989.</ref>

By contrast, csh could evaluate the expression directly, which made it faster.  It also claimed better readability:  Its expressions used a [[Formal grammar|grammar]] and a set of operators mostly copied from C, none of its keywords were reversed and the overall style was also more like C.

Here is a second example, comparing scripts that calculate the first 10 powers of 2.

{{column
|width=60em
|1=
Bourne shell
<syntaxhighlight lang="bash">
#!/bin/sh
i=2
j=1
while [ $j -le 10 ]
do
   echo '2 **' $j = $i
   i=`expr $i '*' 2`
   j=`expr $j + 1`
done
</syntaxhighlight>
|2=
C shell
<syntaxhighlight lang="csh">
#!/bin/csh
set i = 2
set j = 1
while ( $j <= 10 )
   echo '2 **' $j = $i
   @ i *= 2
   @ j++
end
</syntaxhighlight>
}}

Again because of the lack of an expression grammar, the sh script uses [[command substitution]] and the [[expr]] command. (Modern [[POSIX shell]] ''does'' have such a grammar: the statement could be written {{code|1=i=$((i * 2))}} or {{code|1=: "$((i *= 2))"}}.)

Finally, here is a third example, showing the differing styles for a [[switch statement]].

{{column
|width=60em
|1=
Bourne shell
<syntaxhighlight lang="bash">
#!/bin/sh
for i in d*
do
   case $i in
      d?) echo $i is short ;;
      *) echo $i is long ;;
   esac
done
</syntaxhighlight>
|2=
C shell
<syntaxhighlight lang="csh">
#!/bin/csh
foreach i ( d* )
   switch ( $i )
      case d?:
         echo $i is short
         breaksw
      default:
         echo $i is long
   endsw
end
</syntaxhighlight>
}}

In the sh script, "<code>;;</code>" marks the end of each case because sh disallows null statements otherwise.