Functions

In scripts we have the ability to define and call functions. As in any programming language, functions in bash are chunks of code, but there are differences.

In bash, functions are a sequence of commands grouped under a single name, that is the name of the function. Calling a function is the same as calling any other program, you just write the name and the function will be invoked.

We can declare our own function this way:

my_func () {
  # statements
}

my_func # call my_func

We must declare functions before we can invoke them.

Functions can take on arguments and return a result — exit code. Arguments, within functions, are treated in the same manner as arguments given to the script in non-interactive mode — using positional parameters. A result code can be returned using the return command.

Below is a function that takes a name and returns 0, indicating successful execution.

# function with params
greeting () {
  if [[ -n $1 ]]; then
    echo "Hello, $1!"
  else
    echo "Hello, unknown!"
  fi
  return 0
}

greeting Denys  # Hello, Denys!
greeting        # Hello, unknown!

We already discussed exit codes. The return command without any arguments returns the exit code of the last executed command. Above, return 0 will return a successful exit code. 0.

Debugging

The shell gives us tools for debugging scripts. If we want to run a script in debug mode, we use a special option in our script's shebang:

#!/bin/bash options

These options are settings that change shell behavior. The following table is a list of options which might be useful to you:

Short	Name	Description
`-f`	noglob	Disable filename expansion (globbing).
`-i`	interactive	Script runs in interactive mode.
`-n`	noexec	Read commands, but don't execute them (syntax check).
	pipefail	Make pipelines fail if any commands fail, not just if the final command fail.
`-t`	—	Exit after first command.
`-v`	verbose	Print each command to `stderr` before executing it.
`-x`	xtrace	Print each command and its expanded arguments to `stderr` before executing it.

For example, we have script with -x option such as:

#!/bin/bash -x

for (( i = 0; i < 3; i++ )); do
  echo $i
done

This will print the value of the variables to stdout along with other useful information:

$ ./my_script
+ (( i = 0 ))
+ (( i < 3 ))
+ echo 0
0
+ (( i++  ))
+ (( i < 3 ))
+ echo 1
1
+ (( i++  ))
+ (( i < 3 ))
+ echo 2
2
+ (( i++  ))
+ (( i < 3 ))

Sometimes we need to debug a part of a script. In this case using the set command is convenient. This command can enable and disable options. Options are turned on using - and turned off using +:

#!/bin/bash

echo "xtrace is turned off"
set -x
echo "xtrace is enabled"
set +x
echo "xtrace is turned off again"

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