Bash

Variables

name="Ada"                       # no spaces around =
echo "$name"                     # Ada
readonly pi=3.14159              # cannot be reassigned

# Command substitution
current_dir=$(pwd)
files=$(ls "$HOME")

# Arithmetic
count=10
count=$((count + 1))             # 11
echo $((5 * 3))                  # 15

• Quote variables: "$name" prevents word splitting and globbing.
• Use $( ) for command substitution; avoid backticks.
• No spaces around = when assigning.

Quoting

single='literal $HOME'       # no expansion
double="expanded $HOME"      # expands variables and commands

• Single quotes: everything literal.
• Double quotes: variable and command substitution happen; globbing is disabled.
• Always quote variable expansions in most contexts.

Conditionals

if [ "$name" = "Ada" ]; then
    echo "hi Ada"
elif [ -f /tmp/file ]; then
    echo "file exists"
else
    echo "other"
fi

# Test operators
#   [ -f file ]  file exists and is a regular file
#   [ -d dir ]   directory exists
#   [ -n "$s" ]  string non-empty
#   [ -z "$s" ]  string empty
#   [ "$a" = "$b" ]   string equality
#   [ "$a" -eq "$b" ] integer equality
#   [ "$a" -gt "$b" ] integer greater-than

# Numeric test with [[ ]]
if [[ $count -gt 5 ]]; then
    echo "big"
fi

• [ ] is the POSIX test command; [[ ]] is a Bash extension (pattern matching, no word-splitting).
• Terminate if with fi.

Loops

for i in 1 2 3; do
    echo "$i"
done

for f in *.txt; do
    echo "processing $f"
done

i=0
while [ "$i" -lt 5 ]; do
    echo "$i"
    i=$((i + 1))
done

i=0
until [ "$i" -ge 5 ]; do
    echo "$i"
    i=$((i + 1))
done

• for iterates over a list (expansions happen first).
• while repeats while the test is true.
• Use break to exit a loop, continue to skip to the next iteration.

Functions

greet() {
    local name="$1"
    echo "Hello, $name"
}

add() {
    local a=$1 b=$2
    echo $((a + b))
}

greet "Ada"          # "Hello, Ada"
result=$(add 2 3)    # result = 5

# Return an exit status, not a value
fail() {
    return 1
}

• $1, $2, … are positional parameters; $@ is all of them.
• local scopes a variable to the function.
• Functions communicate results via stdout; return sets the exit status (0 = success).

Positional Parameters and Special Variables

script_name=$0
first_arg=$1
all_args=$@        # each argument separately quoted
arg_count=$#
last_status=$?     # exit status of the last command
pid=$$

• $? holds the exit status of the last executed command.
• $# is the number of arguments.
• $@ vs $*: $@ keeps each argument separate when quoted.

Exit Codes

# A command's exit status: 0 = success, non-zero = failure
if grep -q "needle" file.txt; then
    echo "found"
else
    echo "not found (exit $?)"
fi

# Explicitly exit with a status
exit 1   # signal an error

# Chain only on success/failure
cmd1 && cmd2      # run cmd2 only if cmd1 succeeds
cmd1 || cmd2      # run cmd2 only if cmd1 fails

Arrays

fruits=("apple" "banana" "cherry")
echo "$fruits"        # first element: apple
echo "${fruits[1]}"   # banana
fruits+=("date")      # append
echo "${#fruits[@]}"  # length 4

for fruit in "${fruits[@]}"; do
    echo "$fruit"
done

• Declare with parentheses; index with [n].
• "${arr[@]}" expands to each element as a separate word.
• ${#arr[@]} is the element count.

Parameter Expansion

name="Ada Lovelace"

echo "${name#Ada}"      # remove prefix "Ada"   -> " Lovelace"
echo "${name%Lovelace}" # remove suffix "Lovelace" -> "Ada "
echo "${name// /_}"     # replace all spaces with "_"
echo "${name:-default}" # use "default" if name is empty/unset
echo "${name:0:3}"      # first 3 characters -> "Ada"
len=${#name}            # string length

• ${var:-def} provides a default for empty/unset variables.
• ${#var} is the string length.
• Pattern-based prefix/suffix removal and global replacement are available.

Shell Safety

# Fail fast on errors
set -e          # exit on any command that returns non-zero
set -u          # exit on use of an unset variable
set -o pipefail # a pipeline fails if any command in it fails

# Common combination at the top of a script
set -euo pipefail

• set -e can cause surprising early exits; test carefully.
• Prefer local in functions and quote expansions.

Conditionals on Commands

if command -v curl >/dev/null 2>&1; then
    echo "curl is installed"
fi

# Run a command only if its prerequisite exists
mkdir -p build && cmake --install build

References

• Bash Reference Manual
• Google Shell Style Guide
• explainshell.com