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List folders containing only PNGs

list block devices
Shows all block devices in a tree with descruptions of what they are.

tar directory and compress it with showing progress and Disk IO limits
tar directory and compress it with showing progress and Disk IO limits. Pipe Viewer can be used to view the progress of the task, Besides, he can limit the disk IO, especially useful for running Servers.

uniq without pre-sorting
Reads stdin, and outputs each line only once - without sorting ahead of time. This does use more memory than your system's sort utility.

Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

list all opened ports on host

determine if tcp port is open
for udp nmap -sU -p 80 hostname

Detect illegal access to kernel space, potentially useful for Meltdown detection
Based on capsule8 agent examples, not rigorously tested

Emulating netcat -e (netcat-traditional or netcat-openbsd) with the gnu-netcat
Then just nc servername 2600 and ./script.sh kill the client with ctrl+c. You can reconnect several times. kill the server with exit

Determine if a command is in your $PATH using POSIX
it is generally advised to avoid using which(1) whenever possible. which(1) is usually a csh(1) script, or sometimes a compiled binary. It's output is highly variable from operating system to operating system, so platform independent scripts could become quite complicated with the logic. On HP-UX 10.20, for example, it prints "no bash in /path /path /path ..."; on OpenBSD 4.1, it prints "bash: Command not found."; on Debian (3.1 through 5.0 at least) and SuSE, it prints nothing at all; on Red Hat 5.2, it prints "which: no bash in (/path:/path:...)"; on Red Hat 6.2, it writes the same message, but on standard error instead of standard output; and on Gentoo, it writes something on stderr. And given all these differences, it's still variable based on your shell. This is why POSIX is king. See http://mywiki.wooledge.org/BashFAQ/081 for more ways on avoiding which(1).


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