Bash Notes

echo

echo "Hello World with new line"

echo -n "Hello World without new line"

while

while [ $bool ] do

# ...Code...

done

for

for (( i = 0; i < 10; i++ )) do

# ...Code...

done

if

if [ $bool ] ; then
    # Do something
elif [ $other_bool ] ; then
    # Do something else
else
    # Do something else
fi

Take input

read a
echo $a

Arguments

Number of arguments: $#

Nth Argument: $n (n = [1-9])

Functions

function name(){

}

Read a file

file="book.txt"

while read line; do
    echo $line
done < $file

Read a directory

for i in [dir]/* ; do
    echo "$i"
done

if tests

arg = $1

if [ -e $arg ] ; then
    echo "$arg exists"
fi

if [ -f $arg ] ; then
    echo "$arg is a file"
elif [ -d $arg ] ; then
    echo "$arg is a directory"
else
    echo "$arg is something else"
fi

if [ $# -ne 1 ] ; then
    echo "I need only one argument" 1>&2
    exit 1;
fi

if [ ! -f $1 ] ; then
    echo "Argument is not a file" 1>&2
    exit 2;

sleep

echo "Wait for 5 seconds"
sleep 5
echo "Done"

cut

cut -d [seperation_character] -f[field_num]

cut -d " " -f1

sort

• [expression] | sort -k[num] -t [char]

Sort the lines of the output of the [expression] based on the [num] field after sererating them based on the [char]

uniq

Before uniq always sort

Removes concurrent duplicate lines

-c adds a prefix of the number of occurences

ls | sort | uniq -c

tr

• tr [first_set] [second_set]

Replaces the first set of characters with the second one

tr [:lower:] [:upper:]

• tr -s [character]

Replace each sequence of a repeated character that is listed in the last specified SET, with a single occurrence of that character

tr -s " "

Inline code

a=[command]

The result of the [command] goes into the variable a

head

[expression] | head -n[num]

Print the first [num] of lines of the [expression]‘s output

tail

[expression] | tail -n[num]

Print the last [num] of lines of the [expression]‘s output

Error message

echo [message] 1>&2; exit [num!=0]

if [ $# -ne 1 ] ; then
    echo "One argument needed" 1>&2;
    exit 1
fi

egrep

egrep [options] PATTERN [FILE…]

Options

-v reverse of the pattern

--color adds color to matches

-c count the number of matches

-i ignores case