Basics

Running a script

Before you can run a script, you have to set the execution bit on it with chmod:

$ chmod +x script_name
$ ./script_name

Arguments

Shell arguments are 0-indexed, and are always preceded with the $ symbol. For example, $0 is the shell itself, and $1 is the first argument passed to the shell.

You can process all CLI args at once with the following commands. Notice the different behavior when each command is wrapped in double quotes (""):

$*

Specifies all command arguments.

$@

Specifies all command arguments.

"$*"

Takes the entire argument list as one argument, with a space between each value.

"$@"

Takes the entire argument list as separates it into individual arguments.

Look into the shift command.

Variables

A variable is a user-defined value that points to data. Define variables with the following syntax:

$ var='value to assign'

# access with '$'
$ echo $var
$ print $var
$ cat $var_file

Symbol commands

Quotes and tics

Backticks, double-, and single-quotes have different use cases:

• single (''): When you want to use the literal text in the variable or command statement and do not want character or command substution.
• double (""): Allow white space, and character or command substitution.
• backticks (` `): Execute a command or script and have its ouput substituted.

Functions

There are a number of ways to define functions in bash. You should use the function keyword and no parentheses to emphasize that Bash functions do not accept paramters. Additionally, you can put the curly braces on separate lines. Make sure that you define a function before you can use it:

# signature options:
function Helper() {}
function Helper {}
Helper () {}

# preferred
function func_name {
    # commands
}

Control flow

The following sections describe basic control flow syntax.

if…then…

End the if block with fi (if spelled backwards):

# if...then

if [ test_command ]
then
    # command
fi

# if...then...else

if [ test_command ]
then
    # command
else
    # command
fi

# if...then...elif...(else)

if [ test_command ]
then
    # command
elif
    # command
else
    # command
fi

Loops

Indicate that a loop is complete with the done keyword:

# for...in
for loop_var in arg_list
do
    # commands
done

# while
while test_condition_is_true
do
    # commands
done

# until
until test_condition_is_true
do
    # commands
done

Case statement

The case statement is the Bash equivalent to the switch statement in other languages. Indicate the end of command execution for each case with double semi-colons (;;). The default (“fall-through”) case is indicated with the wildcard (*) character:

case $var in

match_1)
    # commands for match_1
    ;;
match_2)
    # commands for match_2
    ;;
match_3)
    # commands for match_3
    ;;
*)
    # commands for default (no match)
    ;;
esac

Control keywords

You can use the following keywords when you want to control loop execution apart from the conditions:

break

Terminates the execution of a loop.

continue

Passes control of a loop to the next iteration.

exit

Exits the entire script.

return

Sends back data, result, or return code to the calling script.

Here documents

A here document provides values to a script without user action. It is a multiline string or file literal that sends input streams to other commands and programs.

The here document must contain data in the exact format that the script expects, or it fails. For example, if there are extra spaces, the script fails.

I need to revisit this. For much better examples, see the Advanced Bash scripting guide.

Traps (termination signals)

When a program terminates before its supposed to, the computer sends an exit symbol. This is called a trap. You can view them all with kill -l:

$ kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 ...

Processes

The following table shows common ps commands: