Bash

Use output of one program, one by one, as an argument to another:

ls | xargs -n 1 rm $1

Count the number of files per directory:

for i in ./*; do echo $i; find $i |wc -l; done

Split lines of output:

command | sed -e "s/delimiter/\n/g" | tail -x

Read first n lines from file:

line=$(head -n 1 filename)

Assign output of a shell command to a variable:

pwd=`pwd`

or:

pwd=$(pwd)

Test if file or directory exists:

if [ -f /tmp/foo.txt ]
then
    echo the file exists
fi

How to write one-liners:

if [ -f config/some.yml.example ]; then mv config/some.yml.example config/some.yml; fi

if [ -d /tmp ]
then
    echo the directory exists
fi

How to repeat command n times:

vim ~/.bash-aliases

repeat() {
    n=$1
    shift
    while [ $(( n -= 1 )) -ge 0 ]
    do
        "$@"
    done
}

After you source ~/.bash-aliases you will be able to run repeat 5 echo hello.

Echo something to a file as root:

sudo sh -c "echo 'something' >> /etc/privilegedfile"

Find and replace across files:

find . -name "*.rb" -print | xargs sed -i 's/foo/bar/g'

Find and replace in a single file:

sudo sed -i 's/#START=yes/START=yes/g' /etc/default/beanstalkd

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