Sourcing Files to Share Functions

Sourcing a File to Import Functions

source filename (or the equivalent shorthand . filename) executes another file's commands directly within the current shell, so any functions or variables it defines become available afterward in the calling script. This is the standard mechanism for splitting shared logic into a reusable library file.

Example: Sourcing a File to Import Functions

#!/bin/bash
cat <<'EOF' > mathlib.sh
add() {
    echo $(( $1 + $2 ))
}
multiply() {
    echo $(( $1 * $2 ))
}
EOF
source mathlib.sh
echo "3 + 4 = $(add 3 4)"
echo "3 * 4 = $(multiply 3 4)"

The . Shorthand

The single dot . is a POSIX-standard shorthand for source and behaves identically; . ./mathlib.sh is exactly equivalent to source ./mathlib.sh. The dot form is more portable to shells other than Bash, while source is more explicit and readable.

Example: The . Shorthand

#!/bin/bash
cat <<'EOF' > greetlib.sh
say_hello() {
    echo "Hello from the sourced library!"
}
EOF
. ./greetlib.sh
say_hello

Variables Are Shared Too

Sourcing shares not just functions but also plain variables defined in the sourced file, since the file's contents run directly in the current shell's own environment. This makes sourcing useful for sharing configuration values as well as reusable functions.

Example: Variables Are Shared Too

#!/bin/bash
cat <<'EOF' > config.sh
APP_NAME="MyTool"
VERSION="1.2.0"
EOF
source config.sh
echo "$APP_NAME version $VERSION"

Sourcing Runs in the Current Shell, Not a Subshell

Unlike bash script.sh or ./script.sh (which start a brand-new child shell that exits independently), source script.sh executes entirely within the calling shell. This is why an exit command inside a sourced file terminates the whole calling script rather than just returning control to it.

Warning: Avoid putting a bare exit in a file meant to be sourced -- use return instead so it only stops the sourced file's own execution, not the whole calling shell.

Example: Sourcing Runs in the Current Shell, Not a Subshell

#!/bin/bash
cat <<'EOF' > safe_lib.sh
lib_function() {
    echo "Library function ran without exiting the caller"
}
EOF
source safe_lib.sh
lib_function
echo "Caller script continues normally after sourcing"