10 Lesser‑Known Linux Commands Every Sysadmin Should Master

pgrep

Searches for processes by name or other attributes and prints their PIDs.

$ pgrep -u hchen
22441
22444

Equivalent to ps -ef | egrep '^hchen' | awk '{print $2}'.

pstree

Displays the process hierarchy as a tree, making parent‑child relationships obvious.

$ pstree
init-+-acpid
     |-auditd-+-python
     |        `-{auditd}
     |-automount---4*[{automount}]
     |-backup.sh---sleep
     ...

bc

Arbitrary‑precision calculator useful for mathematical operations. Example script sqrt computes square roots:

#!/bin/bash
if [ $# -eq 0 ]; then
    echo 'Usage: sqrt number'
    exit 1
else
    echo -e "sqrt($1)
quit
" | bc -q -i
fi

$ ./sqrt 36
6
$ ./sqrt 2.0000
1.4142
$ ./sqrt 10.0000
3.1622

split

Divides a large file into fixed‑size chunks.

$ split -b 50m largefile.tar.gz LF_
$ ls -l LF_*
LF_aa 52428800
LF_ab 52428800
... 
LF_ai 17344374

Recombine the pieces:

$ cat LF_* > largefile.tar.gz

nl

Numbers the lines of a file, similar to cat -n.

$ 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.
     ...

mkfifo

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

$ mkfifo /tmp/hchenpipe
$ ls -l /tmp/hchenpipe
prw-rw-r-- 1 hchen hchen 0 ... hchenpipe

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

$ head /tmp/hchenpipe
...output of ls -al...

ldd

Lists the shared libraries required by an executable.

$ ldd /usr/bin/java
linux-gate.so.1 => (0x00cd9000)
libgij.so.7rh => /usr/lib/libgij.so.7rh (0x00ed3000)
... /lib/ld-linux.so.2 (0x00214000)

col

Converts formatted man‑page output to plain text. Useful when piping man to a file.

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

xmlwf

Validates an XML document, reporting well‑formedness errors such as mismatched tags.

$ curl 'https://coolshell.cn/?feed=rss2' > cocre.xml
$ xmlwf cocre.xml
cocre.xml:13:23: mismatched tag

lsof

Lists open files; when combined with grep it can show network sockets.

$ lsof | grep TCP
httpd 548 apache 4u IPv6 14300967 TCP *:http (LISTEN)
sshd 1764 root 3u IPv6 4993 TCP *:ssh (LISTEN)
...