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Bash

Set memory limit (in KB) for programs run in this terminal:

ulimit -v 100000

Show all limits currently set:

ulimit -a

First line of script (*.sh) should be something like:

#!/bin/bash
#!/usr/bin/python
#!/path/to/my/cool/interpreter

Loop through numbers:

for i in {5..10}; do
  echo $i
done

Loop through all lines in a file

while read line; do
  echo -n "comparing $line and foo... "
  if [[ "$line" == "foo" ]]; then
    echo "They're equal!"
  else
    echo "They're not equal!"
  fi
done < myfile.txt

Loop through all files matching a given pattern:

for path in ./articles/*.text; do
  echo $path
done

Using variables and different kinds of quotes:

var=World
echo "Hello $var"
date
i=is
echo "Today $i date"
echo "Today $i 'date'"
echo "Today $i `date`"
echo 'Today $i date'
echo 'Today $i `date`'

Replace patterns with sed:

echo "http%3A%2F%2Fwww.w3schools.com%2Fcss%2Fcss_margin.asp" | sed -e 's/%3A/:/g' -e 's/%2F/\//g'

Redirect/append output to a file:

echo "Hello World!" > out.txt   
echo "This is line 2, appended to line 1" >> out.txt

Clear contents of a file:

> out.txt

Make shell script executable:

chmod a+x ./test.sh

Extract all lines from myfile.txt starting with four spaces:

grep "    " <./myfile.txt