10 Lesser-Known Linux Commands Every Sysadmin Should Master

1) pgrep

The pgrep command lists process IDs matching a pattern, similar to ps -ef | egrep '^hchen' | awk '{print $2}'.

$ pgrep -u hchen
22441
22444

2) pstree

pstree

displays the process hierarchy in a tree format.

[hchen@RHELSVR5 ~]$ pstree
init-+-acpid
     |-auditd-+-python
     ... (truncated tree output) ...

3) bc

bc

is a high‑precision calculator useful for arithmetic such as square‑root calculations. Example script sqrt:

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

Usage:

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

4) split

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 LF_aa
-rw-r--r-- 1 hchen hchen 52428800 LF_ab
... (other parts) ...

Reassemble with:

# cat LF_* > largefile.tar.gz

5) nl

nl

numbers the lines of a file, similar to cat but with 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.
 ...

6) mkfifo

mkfifo

creates a named pipe.

[hchen@RHELSVR5 ~]# mkfifo /tmp/hchenpipe

[hchen@RHELSVR5 ~]# ls -l /tmp
prw-rw-r-- 1 hchen hchen 0 Apr 5 18:58 hchenpipe

Writing to the pipe blocks until another process reads from it:

# ls -al > /tmp/hchenpipe   # blocks
# head /tmp/hchenpipe      # reads and unblocks

7) ldd

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)
... (other libraries) ...

8) col

col

converts man page output to plain text.

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

9) xmlwf

xmlwf

validates an XML file.

[hchen@RHELSVR5 ~]# curl 'https://coolshell.cn/?feed=rss2' > cocre.xml
... download output ...
[hchen@RHELSVR5 ~]# xmlwf cocre.xml
cocre.xml:13:23: mismatched tag

10) lsof

lsof

lists open files; useful for finding which processes are using network ports.

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