add cheatsheet for df iwconfig ls and rm

cheatsheets/df

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+# Printout disk free space in a human readable format
+df -h

cheatsheets/iwconfig

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+# Display wireless settings of the first wireless adapter
+iwconfig wlan0
+
+# Take down / up the wireless adapter
+iwconfig wlan0 txpower {on|auto|off}
+
+# Change the mode of the wireless adapter
+iwconfig wlan0 mode {managed|ad-hoc|monitor}

cheatsheets/ls

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+# Displays everything in the target directory
+ls path/to/the/target/directory
+
+# Displays everything including hidden files
+ls -a
+
+# Displays all files, along with the size (with unit suffixes) and timestamp
+ls -lh 
+
+# Display files, sorted by size
+ls -S

cheatsheets/rm

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+# Remove files and subdirs
+rm -r path/to/the/target/
+
+# Ignore non existent files 
+rm -f path/to/the/target