10 Lesser-Known Linux Commands Every Sysadmin Should Master

Below are ten useful Linux commands you may not have used before.

1) pgrep

Lists process IDs matching criteria, similar to ps -ef | egrep '^user' | awk '{print $2}'.

$ pgrep -u hchen
22441
22444

2) pstree

Displays the process hierarchy as a tree.

[hchen@RHELSVR5 ~]$ pstree
init-+-acpid
     |-auditd-+-python
     |        `-{auditd}
     |-automount---4*[{automount}]
     |-backup.sh---sleep
     |-dbus-daemon
     |-events/0
     |-events/1
     |-hald---hald-runner---hald-addon-acpi
     |-httpd---10*[httpd]
     |-irqbalance
     |-khelper
     |-klogd
     |-ksoftirqd/0
     |-ksoftirqd/1
     |-kthread-+-aio/0
     |         |-aio/1
     |         |-ata/0
     |         |-ata/1
     |         |-ata_aux
     |         |-cqueue/0
     |         |-cqueue/1
     |         |-kacpid
     |         |-kauditd
     |         |-kblockd/0
     |         |-kblockd/1
     |         |-kedac
     |         |-khubd
     |         |-6*[kjournald]
     |         |-kmirrord
     |         |-kpsmoused
     |         |-kseriod
     |         |-kswapd0
     |         |-2*[pdflush]
     |         |-scsi_eh_0
     |         |-scsi_eh_1
     |         |-xenbus
     |         `-xenwatch
     |-migration/0
     |-migration/1
     |-6*[mingetty]
     |-3*[multilog]
     |-mysqld_safe---mysqld---9*[{mysqld}]
     |-smartd
     |-sshd---sshd---sshd---bash---pstree
     |-svscanboot---svscan-+-3*[supervise---run]
                     |-supervise---qmail-send-+-qmail-clean
                     |                        |-qmail-lspawn
                     |                        `-qmail-rspawn
                     `-2*[supervise---tcpserver]
     |-syslogd
     |-udevd
     |-watchdog/0
     |-watchdog/1
     `-xinetd

3) bc

Arbitrary‑precision calculator useful for mathematical operations such as square roots.

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

Example usage:

[hchen@RHELSVR5]$ ./sqrt 36
6
[hchen@RHELSVR5]$ ./sqrt 2.0000
1.4142
[hchen@RHELSVR5]$ ./sqrt 10.0000
3.1622

4) split

Divides a large file into smaller pieces.

[hchen@RHELSVR5 applebak]# ls -l largefile.tar.gz
-rw-r--r-- 1 hchen hchen 436774774 04-17 02:00 largefile.tar.gz
[hchen@RHELSVR5 applebak]# split -b 50m largefile.tar.gz LF_
[hchen@RHELSVR5]# ls -l LF_*
-rw-r--r-- 1 hchen hchen 52428800 05-10 18:34 LF_aa
-rw-r--r-- 1 hchen hchen 52428800 05-10 18:34 LF_ab
... (additional parts) ...

Reassemble with:

cat LF_* > largefile.tar.gz

5) nl

Numbers lines of a file, similar to cat but adds line numbers.

[hchen@RHELSVR5 include]# nl stdio.h | head -n 10
     1  /* Define ISO C stdio on top of C++ iostreams.
     2  Copyright (C) 1991,1994-2004,2005,2006 Free Software Foundation, Inc.
     3  This file is part of the GNU C Library.
     4  The GNU C Library is free software; you can redistribute it and/or
     5  modify it under the terms of the GNU Lesser General Public
     6  License as published by the Free Software Foundation; either
     7  version 2.1 of the License, or (at your option) any later version.
     8  The GNU C Library is distributed in the hope that it will be useful,

6) mkfifo

Creates a named pipe (FIFO) for inter‑process communication.

[hchen@RHELSVR5 ~]# mkfifo /tmp/hchenpipe
[hchen@RHELSVR5 ~]# ls -l /tmp
prw-rw-r-- 1 hchen hchen 0 05-10 18:58 hchenpipe

Write to the pipe (blocks until read): ls -al > /tmp/hchenpipe Read from another shell to unblock:

head /tmp/hchenpipe

7) ldd

Shows the shared libraries required by an executable.

[hchen@RHELSVR5 ~]# ldd /usr/bin/java
        linux-gate.so.1 =>  (0x00cd9000)
        libgij.so.7rh => /usr/lib/libgij.so.7rh (0x00ed3000)
        libgcj.so.7rh => /usr/lib/libgcj.so.7rh (0x00ed6000)
        libpthread.so.0 => /lib/i686/nosegneg/libpthread.so.0 (0x00110000)
        ... (additional libraries) ...

8) col

Converts man pages to plain text.

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

9) xmlwf

Validates an XML document for well‑formedness.

[hchen@RHELSVR5 ~]# curl 'https://coolshell.cn/?feed=rss2' > cocre.xml
[...download output...]
[hchen@RHELSVR5 ~]# xmlwf cocre.xml
[hchen@RHELSVR5 ~]# perl -i -pe 's@<link>@<br>@g' cocre.xml
[hchen@RHELSVR5 ~]# xmlwf cocre.xml
cocre.xml:13:23: mismatched tag

10) lsof

Lists open files and the processes that opened them.

[root@RHELSVR5 ~]# lsof | grep TCP
httpd       548    apache    4u     IPv6   14300967    TCP *:http (LISTEN)
httpd       548    apache    6u     IPv6   14300972    TCP *:https (LISTEN)
sshd       1764      root    3u     IPv6       4993    TCP *:ssh (LISTEN)
... (additional lines) ...