Bash

Bash (Bourne Again SHell) is a command language and Unix shell. Bash is a superset of Bourne shell (sh): commands that work in sh, also work in bash.

When to Use Bash

The Google Shell Style Guide gives advice on when to use shell.

Shell should only be used for small utilities or simple wrapper scripts.

  • If you're mostly calling other utilities and are doing relatively little data manipulation, shell is an acceptable choice for the task.
  • If you are writing a script that is more than 100 lines long, you should probably be writing it in Python instead.

Variables

# Use uppercase for constants.
NAME="Elliot"
echo "Hi, ${NAME}"

# Use snake_case for variables.
count=0
(( count++ ))
echo ${count}

Prefer ${var} over $var for variable expansion.

Functions

# Use snake_case for function names.
greet() {
    # Declare function-specific variables with 'local'.
    local name="$1"
    echo "Hello, ${name}"
}

greet "Elliot"

Arguments

Use special parameters to access parameters given to the function.

$# Number of arguments
$@ All arguments
$1 First argument
$2 Second argument

Control Flow

if [[ expression ]]; then
    ...
else
    ...
fi

Conditionals

[[ NUM -eq NUM ]] Numbers equal
[[ STRING == STRING ]] Strings equal
[[ ! EXPR ]] Not
[[ X ]] && [[ Y ]] And
[[ X ]] ❘❘ [[ Y ]] Or
[[ -z STRING ]] Empty string
[[ -e FILE ]] File exists
[[ -d FILE ]] Directory exists

Loops

for i in {1..5}; do
    ...
done

Command Substitution

Command substitution is used to insert the output of one command into a second command. 

today=$(date)
echo "${today}"

Style

Start each file with a shebang and a description the contents. 

#!/bin/bash
#
# Display an example.

Executables (chmod +x file) should have no extension.