sed

_$: sed -i 's/old/new/g' file.txt           # Replace 'old' with 'new'
_$: sed -i 's old new g' file.txt           # Replace 'old' with 'new' using whitespace as separator
_$: sed -i 's/\\bold\\b/new/g' file.txt     # Replace word 'old' with word 'new'
_$: sed -i 's/ /\n/g' ./file                # Replace whitespace with new lines
_$: sed -i 's/\n//g' ./file                 # Remove newlines
_$: sed -i.bak 's/\n//g' ./file             # Remove newlines saving a backup in  file.bak
_$: sed -i 's <[^/] \n& g' file             # Replace <pattern with \n<pattern
_$: sed -i.bak '/pattern/d' file            # Delete all lines containing 'pattern'
_$: sed -i '/^[ ]*$/d' file                 # Delete all lines with just whitespaces
_$: sed -i 's/^[ \t]*//' file               # Delete whitespaces at the beginning
_$: sed -i 's/[ \t]*$//' file               # Delete whitespaces at the end
_$: sed -i 's/^[ \t]*//;s/[ \t]*$//' file   # Delete whitespaces at the beginning and at the end

_$: grep -rl 'old' ./* | xargs sed -i 's/old/new/g'
_$: grep -rli 'old' --include="*.py" ./* | xargs sed -i 's/old/new/g'

_$: sed -i '/^$/d' $1               # Delete empty lines

_$: sed -n 1,250p bigfile > header  # Show lines 1 to 250 (inclusive)