3.1 What Variables Are Used For

     

3.1 What Variables Are Used For

In general it is a good idea to use variables to represent external programs. This allows users of the makefile to more easily adapt the makefile to their specific environment. For instance, there are often several versions of awk on a system: awk , nawk , gawk . By creating a variable, AWK , to hold the name of the awk program you make it easier for other users of your makefile . Also, if security is an issue in your environment, a good practice is to access external programs with absolute paths to avoid problems with user 's paths. Absolute paths also reduce the likelihood of issues if trojan horse versions of system programs have been installed somewhere in a user's path . Of course, absolute paths also make makefile s less portable to other systems. Your own requirements should guide your choice.

Though your first use of variables should be to hold simple constants, they can also store user-defined command sequences such as: ^[1]

^[1] The df command returns a list of each mounted filesystem and statistics on the filesystem's capacity and usage. With an argument, it prints statistics for the specified filesystem. The first line of the output is a list of column titles. This output is read by awk which examines the second line and ignores all others. Column four of df 's output is the remaining free space in blocks.

 DF  = df AWK = awk free-space := $(DF) .  $(AWK) 'NR =  = 2 { print $ }' 

for reporting on free disk space. Variables are used for both these purposes and more, as we will see.