How can I resolve a hostname to an IP address in a Bash script?

You can use getent, which comes with glibc (so you almost certainly have it on Linux). This resolves using gethostbyaddr/gethostbyname2, and so also will check /etc/hosts/NIS/etc:

getent hosts unix.stackexchange.com | awk '{ print $1 }'

Or, as Heinzi said below, you can use dig with the +short argument (queries DNS servers directly, does not look at /etc/hosts/NSS/etc) :

dig +short unix.stackexchange.com

If dig +short is unavailable, any one of the following should work. All of these query DNS directly and ignore other means of resolution:

host unix.stackexchange.com | awk '/has address/ { print $4 }'
nslookup unix.stackexchange.com | awk '/^Address: / { print $2 }'
dig unix.stackexchange.com | awk '/^;; ANSWER SECTION:$/ { getline ; print $5 }'

If you want to only print one IP, then add the exit command to awk's workflow.

dig +short unix.stackexchange.com | awk '{ print ; exit }'
getent hosts unix.stackexchange.com | awk '{ print $1 ; exit }'
host unix.stackexchange.com | awk '/has address/ { print $4 ; exit }'
nslookup unix.stackexchange.com | awk '/^Address: / { print $2 ; exit }'
dig unix.stackexchange.com | awk '/^;; ANSWER SECTION:$/ { getline ; print $5 ; exit }'

With host from the dnsutils package:

$ host unix.stackexchange.com
unix.stackexchange.com has address 64.34.119.12

(Corrected package name according to the comments. As a note other distributions have host in different packages: Debian/Ubuntu bind9-host, openSUSE bind-utils, Frugalware bind.)

I have a tool on my machine that seems to do the job. The man page shows it seems to come with mysql... Here is how you could use it:

resolveip -s unix.stackexchange.com
64.34.119.12

The return value of this tool is different from 0 if the hostname cannot be resolved :

resolveip -s unix.stackexchange.coma
resolveip: Unable to find hostid for 'unix.stackexchange.coma': host not found
exit 2

UPDATE On fedora, it comes with mysql-server :

yum provides "*/resolveip"
mysql-server-5.5.10-2.fc15.x86_64 : The MySQL server and related files
Dépôt         : fedora
Correspondance depuis :
Nom de fichier      : /usr/bin/resolveip

I guess it would create a strange dependency for your script...