Essential Linux Commands with Real‑World Examples

1) pgrep – Lists process IDs matching a pattern. Example:

$ pgrep -u hchen
22441
22444

It is equivalent to ps -ef | egrep '^hchen' | awk '{print $2}' 2) pstree – Displays the process hierarchy as a tree. Example output shows the parent‑child relationships of system services and user processes.

3) bc – An arbitrary‑precision calculator useful for mathematical operations such as square‑root calculations. A sample script sqrt demonstrates how to compute square roots via

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

Running the script yields results like ./sqrt 36 → 6, ./sqrt 2.0000 → 1.4142, etc.

4) split – Divides a large file into smaller chunks. Example:

[hchen@RHELSVR5 applebak]# split -b 50m largefile.tar.gz LF_

Resulting files LF_aa, LF_ab, … can be recombined with cat LF_* > largefile.tar.gz 5) nl – Numbers lines of a file, similar to cat but adds line numbers. Example: nl stdio.h | head -n 10 6) mkfifo – Creates a named pipe for inter‑process communication. Example creation and usage:

mkfifo /tmp/hchenpipe
ls -l /tmp   # shows the pipe
ls -al > /tmp/hchenpipe   # blocks until another process reads
head /tmp/hchenpipe       # reads and unblocks the writer

7) ldd – Lists shared libraries required by an executable. Example for /usr/bin/java shows each linked library and its path.

8) col – Converts formatted man pages to plain text. Example:

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

9) xmlwf – Validates an XML file, reporting malformed tags. Example workflow downloads an RSS feed, runs xmlwf, and fixes a mismatched tag.

10) lsof – Lists open files and network sockets. Example filters TCP listeners and established connections, showing processes such as httpd, sshd, and mysqld.