#pragma section-numbers 2

= 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.

<<TableOfContents>>

<<Anchor(faq1)>>
== 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.

=== /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
    }
 }}}

=== 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.
  ''This should probably be using `ypmatch` instead of scanning the entire output of `ypcat`, but since I don't have any NIS systems configured to do this, I can't test it myself.''

=== 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:"] }'
    }
 }}}

  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
   }}}
  (You may have to change `-f6` to `-f5` in some versions of `dig`.)

  Newer versions of `dig` allow this:
   {{{
      dig +short a "$1" | grep -v '\.$'
   }}}
  The `grep` here is to discard any CNAME records that show up.

=== LDAP - Lightweight Directory Access Protocol ===

 {{{
    # lookup hostname using LDAP
    lookup_ldap () {
       # TODO: add query
    }
 }}}

<<Anchor(faq2)>>
== How can I copy a directory with all files and subdirectories? ==
{{{
    source=/home/$USER
    destination=/tmp/backup

    cd "$source"
    find . -print | cpio -pdmv "$destination"
}}}

This method can be used to receate a directory hierarchy, without files, too:

{{{
    find . -type d -print | cpio -pdmv "$destination"
}}}

If you're copying across a network, this is an indispensable technique:

{{{
    cd /the/source
    tar cf - dir1 dir2 ... | ssh user@host "cd /the/destination && tar xvf -"
}}}

Antique Unix systems could achieve a similar effect using `rsh` (or `remsh` on some platforms) instead of `ssh`.  `rsh` is ''extremely'' deprecated, but if you're stuck on such an old platform, you ''can'' use it  in an emergency.  Just be sure to undo your rhosts stuff after you're done.

Some systems may have [[http://www.samba.org/rsync/|rsync]] and/or [[http://rdiff-backup.nongnu.org/|rdiff-backup]] installed.  These are extremely powerful alternatives to the `tar` over `ssh` pipeline.

''TODO: pax(1) alternatives to tar/cpio''

<<Anchor(faq3)>>
== How can I remove a file with a name starting with '-'? ==
Safest way:

{{{
    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 for virtually all POSIX utilities.

<<Anchor(faq4)>>
== How can I redirect the output of the time(1) command? ==
This is complicated by the fact that there are multiple implementations of `time` as a command.  It might be a standalone program (e.g. `/usr/bin/time`) or it might be a shell keyword with special features.

In Bash, `time` is a special keyword that executes a pipeline (or a simple command) and writes timing information to standard error (file descriptor 2).  It's not possible to directly redirect its output to a file, because the redirections are applied to the pipeline or command that's being timed.  If you are attempting to save `time` output to a file in Bash, please see [[BashFAQ/032|Bash FAQ #32]].

{{{
    # Bash -- this doesn't work
    time ls 2>time.out
}}}

If your `time` command is a standalone program, then it will not have the special powers of a shell keyword.  Redirections will apply to `time` rather than to the command being timed (which must be a simple command, not a compound one or a pipeline).

{{{
    ~$ /usr/bin/time sleep 1 2>time.out
    ~$ cat time.out
    0.00user 0.00system 0:01.00elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
    0inputs+0outputs (0major+174minor)pagefaults 0swaps
}}}

If you want to capture standalone `time` output in a variable, see [[BashFAQ/002|Bash FAQ #2]].

<<Anchor(faq5)>>
== How do I make a lock file in a script? ==
This is covered in [[BashFAQ/045|Bash FAQ #45]] and ProcessManagement.

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CategoryUnix