if.1

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.th IF I 5/2/74
.sh NAME
if \*-  conditional command
.sh SYNOPSIS
.bd if
expr command [ arg ... ]
.sh DESCRIPTION
.it If
evaluates the expression
.it expr,
and if its value is
true,
executes the given
.it command
with the given arguments.
.s3
The following primitives are used to construct
the
.it expr:
.s3
.lp +13 13
\fB\*-r\fR file	true if the file exists and is readable.
.s3
.lp +13 13
\fB\*-w \fRfile	true if the file exists and is writable.
.s3
.lp +13 13
s1 \fB= \fRs2	true
if the strings
.it s1
and
.it s2
are equal.
.s3
.lp +13 13
s1 \fB!= \fRs2	true
if the strings
.it s1
and
.it s2
are not equal.
.s3
.lp +13 13
\fB{ \fRcommand \fB}\fR	The bracketed command is
executed to obtain the
exit status.
Status zero is considered
.it true.
The command must
not
be another
.it if.
.s3
.i0
These primaries may be combined with the
following operators:
.s3
.lp +13 13
\fB!\fR	unary negation operator
.s3
.lp +13 13
\fB\*-a\fR	binary
.it and
operator
.s3
.lp +13 13
\fB\*-o\fR	binary
.it or
operator
.s3
.lp +13 13
\fB( \fRexpr\fB )\fR	parentheses for grouping.
.s3
.i0
.bd \*-a
has higher precedence than
.bd \*-o.
Notice that all the operators and flags are separate
arguments to
.it if
and hence must be surrounded by spaces.
Notice also that parentheses are meaningful
to the Shell and must be escaped.
.sh "SEE ALSO"
sh (I), find (I)
.sh BUGS