Writing scripts

set -e The script will exit early if any line fails

set -o pipefail The script will mark lines as failed if any piped command on it fails

Variables

Variables are declared just like this varname=value but to be referenced need to have a $ before $varname

Double quotes enable variable interpolation, while single quotes don’t. Use with care.

If you don’t use quotes at all and your variable has whitespaces, it will be interpreted as different arguments.

myVar=1

echo "This is my var $myVar"
# This is my var 1
echo 'This is my var $myVar'
# This is my var $myVar

To assign the output of a command to a variable, you need to encapsulate it like myVar=$(echo "here").

# This will error
myVar=echo "here"

# This will set myVar to here
myVar=$(echo "here")

Functions

# Declaration
<functionName>() {
  # Code here
  firstArgument=$1
  # etc
}

# Call
<functionName> <args>

# For example

printSomething(){
  echo "$1 and also $2"
}

printSomething "text" "other text"

# will print
# text and also other text

Loops and conditionals

for <varname> in <collection>; do
  # Code here
done

# For example
for file in src/*.md; do
  echo "$file"
done

If is done to work with standard error codes and if a comand exits with anything but a 0, it will evaluate to false.

We can use the -q flag to avoid outputs of anything on the if check

if <command>; then
  # Code here
fi

# For example
if grep -q port "$file"; then
  echo "$file"
fi

Or to compare expresions

if [ <comparison> ]; then
  # Code here
fi

# For example
if [ "$port" -lt 5000 ]; then
  echo "Port is under 5000"
fi

if <condition>; then
  # Code here
elif <another condition>; then
  #Code here
else
  # Code here
fi

To enter an if when a variable is true if [ $VARIABLE ]; won’t work. The correct way is if [ “$VARIABLE” = true ];

Some comparators:

  • <a> -lt <b> less than
  • <a> -gt <b> greater than
  • <a> -eq <b> numeric equal to
  • -f <filename> filename is a file and exists
  • -z "$<variableName>" variableName has zero length (empty string)

More expresions

Doing more than check in the same conditional

You can use the normal boolean operators like:

if [ <comparison> ] && [ <comparison> ]; then
  # Code here
fi

Another option is use the -a and -o expresions:

## Equivalent to &&
if [ <comparison> -a <comparison> ]; then
  # Code here
fi


## Equivalent to ||
if [ <comparison> -o <comparison> ]; then
  # Code here
fi

Or double brackets:

if [[ <comparison> && <comparison> ]]; then
  # Code here
fi

Fail checks

You can check the number of arguments provided to a script with $#.

So a fail check can be

if [ $# -eq 0 ]; then
  echo "Arguments missing"
  exit 64
fi