The 10 Commands we never use.

It takes years maybe decades to master the commands available to you at the Linux shell prompt. Here are 10 that you will have never heard of or used. They are in no particular order. My favorite is mkfifo.

1. pgrep, instead of:

   # ps -ef | egrep '^root ' | awk '{print $2}'
   1
   2
   3
   4
   5
   20
   21
   38
   39
   ...
   

   You can do this:

   # pgrep -u root
   1
   2
   3
   4
   5
   20
   21
   38
   39
   ...
   

2. pstree, list the processes in a tree format. This can be VERY useful when working with WebSphere or other heavy duty applications.

   # pstree
   init-+-acpid
        |-atd
        |-crond
        |-cups-config-dae
        |-cupsd
        |-dbus-daemon-1
        |-dhclient
        |-events/0-+-aio/0
        |          |-kacpid
        |          |-kauditd 
   
        |          |-kblockd/0
        |          |-khelper
        |          |-kmirrord
        |          `-2*[pdflush]
        |-gpm
        |-hald
        |-khubd
        |-2*[kjournald]
        |-klogd
        |-kseriod 
   
        |-ksoftirqd/0
        |-kswapd0
        |-login---bash
        |-5*[mingetty]
        |-portmap
        |-rpc.idmapd
        |-rpc.statd
        |-2*[sendmail]
        |-smartd
        |-sshd---sshd---bash---pstree 
   
        |-syslogd
        |-udevd
        |-vsftpd
        |-xfs
        `-xinetd
   

3. bc is an arbitrary precision calculator language. Which is great. I found it useful in that it can perform square root operations in shell scripts. expr does not support square roots.

   # ./sqrt
   Usage: sqrt number
   # ./sqrt 64
   8
   # ./sqrt 132112
   363
   # ./sqrt 1321121321
   36347
   
   

   Here is the script:

   # cat sqrt
   #!/bin/bash
   if [ $# -ne 1 ]
   then
           echo 'Usage: sqrt number'
           exit 1
   else
           echo -e "sqrt($1)\nquit\n" | bc -q -i
   fi
   

4. split, have a large file that you need to split into smaller chucks? A mysqldump maybe? split is your command. Below I split a 250MB file into 2 megabyte chunks all starting with the prefix LF_.

   # ls -lh largefile
   -rw-r--r--  1 root root 251M Feb 19 10:27 largefile
   # split -b 2m largefile LF_
   # ls -lh LF_* | head -n 5
   -rw-r--r--  1 root root 2.0M Feb 19 10:29 LF_aa
   -rw-r--r--  1 root root 2.0M 
    Feb 19 10:29 LF_ab
   -rw-r--r--  1 root root 2.0M Feb 19 10:29 LF_ac
   -rw-r--r--  1 root root 2.0M Feb 19 10:29 LF_ad
   -rw-r--r--  1 root root 2.0M Feb 19 10:29 LF_ae
   # ls -lh LF_* | wc -l
   126
   

5. nl numbers lines. I had a script doing this for me for years until I found out about nl.

   # head wireless.h
   /*
    * This file define a set of standard wireless extensions
    *
    * Version :    20      17.2.06
    *
    * Authors :    Jean Tourrilhes - HPL
    * Copyright (c) 1997-2006 Jean Tourrilhes, All Rights Reserved. 
   
    */
   
   #ifndef _LINUX_WIRELESS_H
   # nl wireless.h | head
        1  /*
        2   * This file define a set of standard wireless extensions
        3   *
        4   * Version :    20      17.2.06
        5   * 
         6   * Authors :    Jean Tourrilhes - HPL
        7   * Copyright (c) 1997-2006 Jean Tourrilhes, All Rights Reserved.
        8   */
   
        9  #ifndef _LINUX_WIRELESS_H
   

6. mkfifo is the coolest one. Sure you know how to create a pipeline piping the output of grep to less or maybe even perl. But do you know how to make two commands communicate through a named pipe?
   
   

   First let me create the pipe and start writing to it:

   

   Then read from it:

   
7. ldd, want to know which Linux thread library java is linked to?

   # ldd /usr/java/jre1.5.0_11/bin/java
           libpthread.so.0 => /lib/tls/libpthread.so.0 (0x00bd4000)
           libdl.so.2 => /lib/libdl.so.2 (0x00b87000)
           libc.so.6 => /lib/tls/libc.so.6 (0x00a5a000) 
   
           /lib/ld-linux.so.2 (0x00a3c000)
   

8. col, want to save man pages as plain text?

   # PAGER=cat
   # man less | col -b > less.txt
   

9. xmlwf, need to know if a XML document is well formed? (A configuration file maybe..)

   # curl -s 'http://bashcurescancer.com' > bcc.html
   # xmlwf bcc.html
   # perl -i -pe 's@<br/>@<br>@g'  bcc.html
   # xmlwf bcc.html
   bcc.html
   :104:2: mismatched tag
   

10. lsof lists open files. You can do all kinds of cool things with this. Like find which ports are open:

    # lsof | grep TCP
    portmap    2587   rpc    4u     IPv4       5544                 TCP *:sunrpc (LISTEN)
    rpc.statd  2606  root    6u     IPv4       5585                 TCP *:668 (LISTEN)
    sshd       2788  root    3u     IPv6       5991                 TCP *:ssh (LISTEN) 
    
    sendmail   2843  root    4u     IPv4       6160                 TCP badhd:smtp (LISTEN)
    vsftpd     9337  root    3u     IPv4      34949                 TCP *:ftp (LISTEN)
    cupsd     16459  root    0u     IPv4      41061                 TCP badhd:ipp (LISTEN) 
    
    sshd      16892  root    3u     IPv6      61003                 TCP badhd.mshome.net:ssh->kontiki.mshome.net:4661  (ESTABLISHED)
    

    Or find the number of open files a user has. Very important for running big applications like Oracle, DB2, or WebSphere:

    # lsof | grep ' root ' | awk '{print $NF}' | sort | uniq | wc -l
    179
    

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