You’ve heard of:

• Variables
• Conditionals
• Loops
• Some *nix tools
• Functions

Let’s repeat, practice and expand.

:)

Variables

Declaration:

• Strings: NAME="value"
• Numbers: VAR=value

Variables

Usage:

• echo "Hello $NAME."
• echo "Hello ${NAME}."
• VAR=$((VAR + 1))

Variables: Local vs Global

function test {
    VAR="$1"
    echo "$VAR"
}

VAR="a"
echo "$VAR"
test "b"
echo "$VAR"

Variables: Local vs Global

function test {
    local VAR="$1"
    echo "$VAR"
}

VAR="a"
echo "$VAR"
test "b"
echo "$VAR"

Functions

function runCmd {
    local CMD="$1"
    echo "Command: $CMD"
    eval $CMD
    return $?
}

runCmd "ffmpeg -i xxx ..."

Functions

function checkError {
    local VALUE=$1
    local MSG="$2"

    if [ $VALUE -ne 0 ]; then
        echo "ERROR: $MSG"
        exit $VALUE
    fi
}

...
RETVAL=$?
checkError $RETVAL "This went wrong."

Store output of command

TEXT=$(command arg1 arg2 arg3 ...)
echo "$TEXT"

Store output of command

TEXT=$(cat /etc/passwd)
echo "$TEXT"

Store output of function

TEXT=$(myFunction "$X")
echo "$TEXT"

Tests / Conditionals

Bash has 3 different types of comparison operators:

1. File test operators
2. Integer comparison
3. String comparison

Tests / Conditionals

• Check if variable matches string: if [ "$NAME" == "Dragon" ]; then ...; fi

• Check if variable is empty: if [ -z "$NAME" ]; then ...; fi

• Check if parameter is a folder: if [ -d "$SOURCE" ]; then ...; fi

• Check if value is greater-than 1: if [ $VALUE -gt 1 ]; then ...; fi

Comparison operators (numeric)

• -eq : equal to
• -ne : not equal to
• -gt : greater than
• -ge : greater or equal
• -lt : less than
• …

Comparison operators (string)

• =/== : string equal to
• != : string not equal to
• -z : string is NULL
• -n : string is not NULL
• …

More: Other comparison operators (tldp.org)

File test operators

• -e: file exists
• -f: is a file (not a folder or device)
• -d: is a directory
• -s: is not zero size
• …

More: File test operators (tldp.org)

btw: NULL

= The computer void.

• string: length of zero
• device: /dev/null
• character: ASCII character index 0
• …

See: https://en.wikipedia.org/wiki/Null#Computing

Spaces and Quotes

NAME="My Name"
if [ $NAME == "My Name" ]; then ...

Throws the error: line x: [: too many arguments

Spaces and Quotes

Because this is what happens internally:

if [ My Name == "My Name" ], then ...

Comparison Tests

• Does FFmpeg executable exist?
• Is $DIR_IN really a folder?
• Is $VAR empty or not?
• Was the last command successful?
• Is $CHOICE this, this or that?

Combined Tests

# OR = ||
if [[ "$1" == "x" || "$1" == "X" ]]; then
    echo "It's an x (or an X)!"
fi

# AND = &&
if [[ $1 -gt 3 && $1 -lt 7 ]]; then
    echo "Bingo!"
fi

if - then - else if - else

if [ $VAL -eq 1 ]; then
    # Do if VAL=1
elif [ $VAL -eq 2 ]; then
    # Do if VAL=2
elif [ $VAL -eq 3 ]; then
    # Do if VAL=3
else
    # Do if anything else.
fi

Case statement

Multiple choice tests ;)

case "$CHOICE" in
    one)
        ...
    ;;

    two)
        ...
    ;;

    *)
        ...
    ;;
esac

See: Testing and Branching (tldp.org)

Case statement

Return and Exit

Return is to a function what Exit is to a program/script.

And they both allow to return a numeric value, stored in $?

Return True or False

function isDir() {
    if [ -d "$1" ]; then
        # 0 = true
        return 0
    else
        # not 0 = false
        return 1
    fi
}

DIR="$1"
echo "Is $DIR a folder...?"

if isDir "$DIR"; then echo "Yes!"; else echo "No."; fi

String manipulation

${#string}                  Length of $string

${string:position}          Extract substring from $string at $position
${string:position:length}   Extract $length characters substring from $string at $position

${string#substring}         Strip shortest match of $substring from front of $string
${string##substring}        Strip longest match of $substring from front of $string
${string%substring}         Strip shortest match of $substring from back of $string
${string%%substring}        Strip longest match of $substring from back of $string

${string/substring/replacement}     Replace first match of $substring with $replacement
${string//substring/replacement}    Replace all matches of $substring with $replacement
${string/#substring/replacement}    If $substring matches front end of $string, substitute $replacement for $substring
${string/%substring/replacement}    If $substring matches back end of $string, substitute $replacement for $substring

See “Advanced Bash-Scripting Guide (tldp.org)” for additional information.

Extract text

Extract fixed length:

LINE=$(md5sum "$0")
HASH=${LINE:0:32}

echo "Line: '$LINE'"
echo "Hash: '$HASH'"

Extract file suffix

FILE="bla.txt"
NAME="${FILE##*.}"
echo $NAME

> ${string##substring} Strip longest match of $substring from front of $string

Exercise:

DPX - 1 to many

Take 1 image file and copy it n times with subsequent filenames, according to a given naming rule.

Programmer Questions

• How could you do that?
• Which input information do you require?
• Which steps are necessary?
• Which problems could you encounter?
• How could you avoid them?
• Or how could you handle errors if they happen?

Steps?

• Copy one file.
• Repeat n times.
• Input data:
  • Sourcefile
  • Output folder
• Other: define naming rule

Options

For loop with “seq”: for i in $(seq 1 $n); do ... done

While loop: while [ $i -le $n ]; do ... done

Homework 1/2

Write a function that takes an integer as input and increases its value by 1 and returns this number. Echo the incremented value.

Optional: Use this function as in a while loop to count up until 13.

Homework 2/2

Write a function that you can use to check if a given string has the following properties:

Return True if the string matches these conditions, False if otherwise.