UNIX for Dummies Questions & Answers

my querry is about writing makefiles:-

i have gone through some sample make file and noticed that while writing the commands for a particular target..

some times two lines are written with a backslash like

LD_LIBRARY_PATH="/usr/openwin/lib"; export LD_LIBRARY_PATH; \
TK_LIBRARY=$(TK_DIR)/lib/tk8.4; export TK_LIBRARY; \
TCL_LIBRARY=$(TCL_DIR)/lib/tcl8.4; export TCL_LIBRARY; \
PATH=$(TK_DIR)/bin:$(TCL_DIR)/bin:${PATH}; export PATH; \
CFLAGS="$(ALL_INCLUDE)"; export CFLAGS; \
LDFLAGS="$(ALL_LLIB) $(ALL_RLIB)"; export LDFLAGS && \
CC=$(HOST_CC) CXX=$(HOST_CXX) MAKE=$(GMAKE) \
$(SRC_DIR)/configure \
--prefix=$(DEST_DIR


....can please any one explain me whats the significance of using "\" and the "&&"
in the command section of the makefile....does something special happens internally when execution starts

thanks & regards
malay

The \ is just a line continuation character, used to break a long line, for readability.

It's not just cosmetic however. make treats each command line as if it were executed in its own shell. Thus

Code:

cd output
rm *

will not work in make as you would expect. But

Code:

cd output;rm *

will work, as both commands are on a single line, which can be split up by using a backslash, like

Code:

cd output; \
rm *

(Note that you still need the semicolon at the end of the line to be continued, as well as the backslash)

As noted above, the backslash just lets you wrap a long command line.

You have to be sure that the backslash is the final character in that line -- even a single blank space will screw it up -- that set of commands will then be perceived as two separate lines.

The && operator is used to execute additional commands after other commands, but only if the earlier one(s) are successful. The || (double-pipe) operator is the opposite -- the following only executes if the first one fails.

Example:

Code:

thing1 && thing2 && thing3 && thing4

If thing1 works, then thing2 will execute. If thing2 works, thing3 will execute, and so on. But if any one of them fails, that's the end of the road.

Code:

thing1 || otherthing

In this case, thing1 will be executed. If it works, that's the end of the road. If it fails, then otherthing will be executed.