Unix Frequently Asked Questions
This FAQ answers frequent questions related to the Unix operating system. If you have questions related to the ["BASH"] shell, the BashFaq is a better place to look.
1. How can I get the IP address for a host name?
Depending on where the (host, ip_address) relationship is stored, different ways of resolving a host name to an ip address (or vice versa) exist. The most common ones are /etc/hosts, NIS, and DNS. As if that wasn't enough, many systems use more than one method, usually configured in the file /etc/nsswitch.conf.
In the easiest case the system has the getent command, which will use /etc/hosts, NIS, DNS, or whatever name resolution was configured in the right sequence:
$ getent hosts www.shelldorado.com 127.0.0.1 www.shelldorado.com
If we don't have this very useful command (_XXX is there an OpenSource version available?_), we are on our own and have to access the respective database ourselves. Note that we don't show here how to find out which name resolutions are configured, or in which sequence they are used.
1.1. /etc/hosts
# lookup hostname using /etc/hosts lookup_files () { # usage: lookup_files hostname awk '{ for ( i=2; i<=NF; ++i ) if ( $i == "'"$1"'" ) print $1 }' /etc/hosts }
1.2. NIS - Network Information System, also known as Yellow Pages
# lookup hostname using NIS lookup_nis () { ypcat hosts | awk '{ for ( i=2; i<=NF; ++i ) if ( $i == "'"$1"'" ) print $1 }' }
Note: the use of NIS to store hostname-to-IP mappings is deprecated.
1.3. DNS - Domain Name Service
# lookup hostname using DNS lookup_dns () { nslookup "$1" 2>/dev/null | awk '{ cnt [$1]++; val [cnt [$1] $1] = $2 } END { print val ["2Address:"] }' }
TODO: how to use host or dig
Suggestions by Tarax: with bind9-host and dig, following pipelines seem to work quite well. Just replace the nslookup/awk instructions above with one of them... and thank you for this page
host "$1" | cut -d" " -f4 | sort | uniq
dig a +noall $1 +answer | cut -f6 | sort | uniq
1.4. LDAP - Lightweight Directory Access Protocol
# lookup hostname using LDAP lookup_ldap () { # TODO: add query }
2. How can I copy a directory with all files and subdirectories?
src=/home/$USER # source dst=/tmp/backup # destination cd "$src" find . -print | cpio -pdmv "$dst"
This method can be used to receate a directory hierarchy, without files, too:
find . -type d -print | cpio -pdmv "$dst"
3. How can I remove a file with a name starting with '-'?
Either
rm ./-filename
(./ is the name of the current directory). Another way (for newer Unix systems):
rm -- -filename
The "--" argument is a standard way to denote the end of the command line options.
4. How can I redirect the output of the time(1) command?
time is a special command that writes to standard error (file descriptor 2). It's not possible to directly redirect its output to a file, e.g.
time ls 2>time.out
would only redirect the ls standard error output, not the time output. A solution is to either call time in a subshell with redirected output, e.g.
sh -c "time ls" 2>time.out
or use braces:
{ time ls; } 2>time.out