Let's say I'm writing something that depends on external programs, like svn. How do I check for their existence automatically, so I can print a helpful error message when they're absent? Iterating through PATH is possible, but hardly elegant and efficient. Are there cleaner solutions?
I've seen this behavior in a bootstrapping script, though I can't remember where. It looked a little like this:
checking for gcc... yes
If you are using bash, you can use the type builtin:
$ type -f svn
svn is /usr/bin/svn
If you want to use it in a script:
$ type -f svn &>/dev/null; echo $?
0
$ type -f svn_doesnt_exist &>/dev/null; echo $?
1
which? That's what I'd normally use.
which is a program, but type is a shell builtin. Furthermore, type can also find stuff like functions and can cut through aliases. For example, if you alias a command, type will tell you the alias whereas which will fail to find anything.Try to actually call it.
It makes most sense to call it with -V or whatever else option that makes the program report its version; most of the time you want the program to be at least such-and-such version.
If your program is a shell script, which is your friend, too.
PATH or not. An attempt to execute will search PATH anyway, and either run the command or return an error. What security or robustness will you buy doing this by hand?
system()in C or via the command line, basically. And yes, Linux/Unix. (See tag.)