Bash Functions: Arguments, local and Return Values

Functions let you name a block of commands and reuse it.

Define and call#

greet() {
  echo "Hello!"
}
greet
greet

Hello!
Hello!

A function must be defined before it is called.

Arguments#

Inside a function, $1, $2 and $# refer to the function's own arguments, and you call it like a command, with no brackets:

greet() {
  echo "Hello, $1. You are from $2."
}
greet "Asha" "Hyderabad"

Hello, Asha. You are from Hyderabad.

Local variables#

Variables are global by default, even inside a function. Use local to keep them there:

total=100
change() {
  local total=5
  echo "Inside: $total"
}
change
echo "Outside: $total"

Inside: 5
Outside: 100

Always use local for variables inside functions.

Returning values#

return sets only an exit code from 0 to 255, used for success or failure. To give back data, echo it, and capture the output with $(...):

add() {
  echo $(( $1 + $2 ))
}
sum=$(add 7 5)
echo "Sum is $sum"

Sum is 12

Use return to report success or failure, so that the function works in an if:

is_even() {
  (( $1 % 2 == 0 ))
}
for n in 3 4; do
  if is_even "$n"; then
    echo "$n is even"
  else
    echo "$n is odd"
  fi
done

3 is odd
4 is even

The last command's status becomes the function's return value.

check_file() {
  if [[ ! -f "$1" ]]; then
    echo "Missing: $1" >&2
    return 1
  fi
  return 0
}
check_file /etc/hostname && echo "Found"
check_file /nothing || echo "Status was $?"

Found
Missing: /nothing
Status was 1

>&2 sends a message to stderr, which keeps error text out of captured output.

All arguments#

"$@" means all the arguments, each kept as a separate word:

show_all() {
  echo "Count: $#"
  for item in "$@"; do
    echo "- $item"
  done
}
show_all one "two words" three

Count: 3
- one
- two words
- three

Share functions between scripts#

Put common functions in a file, and load it with source:

cat > lib.sh <<'EOF'
log() {
  echo "[$1] $2"
}
EOF
cat > app.sh <<'EOF'
#!/bin/bash
source ./lib.sh
log INFO "Started"
log ERROR "Disk is full"
EOF
bash app.sh

[INFO] Started
[ERROR] Disk is full

Structure of a script#

A good script has settings, then functions, then a main:

cat > tidy.sh <<'EOF'
#!/bin/bash
LIMIT=2

report() {
  echo "Keeping the newest $LIMIT"
}

main() {
  report
  echo "Done"
}

main "$@"
EOF
bash tidy.sh

Keeping the newest 2
Done

Common mistakes#

Practice#

max() {
  if (( $1 > $2 )); then
    echo "$1"
  else
    echo "$2"
  fi
}
echo "Max: $(max 4 9)"

Max: 9

Key takeaway#

Next: Arguments and Exit Codes.