envsubst - Linux

Overview

envsubst is a simple yet powerful command used to substitute environment variables in text files or standard input. It scans the input, replaces occurrences of ${VARIABLE} with the corresponding environment variable value, and prints the result.

Syntax

envsubst [OPTIONS] [TEXTFILE]

Options/Flags

-f FILE, –file=FILE: Use the specified file as input instead of standard input.
-d DELIMITER, –delimiter=DELIMITER: Change the default delimiter from $ to the specified character.
-u, –unset: Remove undefined environment variables from the output.
-e, –expand: Expand nested environment variables.
-s, –silent: Suppress error messages for undefined environment variables.
-v, –version: Print version information and exit.
-h, –help: Print usage information and exit.

Examples

Simple Variable Substitution:

echo "${USER}" | envsubst
# Output: testuser

Multi-Line Substitution:

cat config.txt | envsubst
# Content of config.txt:
# ${DB_HOST}
# ${DB_USER}
# ${DB_PASSWORD}
# Output:
# localhost
# dbuser
# secretpassword

Nested Variable Expansion (with -e):

envsubst -e <<EOF
DB_HOST=${HOST}
DB_USER=${USER}
EOF
# Output:
# DB_HOST=myhost
# DB_USER=testuser

Changing Delimiter (with -d):

echo "${FOO:=bar}" | envsubst -d "#"
# Output:
# bar#=bar

Common Issues

Undefined environment variables may cause errors unless -s or -u is used.
Nesting environment variables may lead to unexpected results unless -e is used.
envsubst can consume a lot of memory when expanding complex expressions or handling large input files.

Integration

With xargs:

find . -type f -exec envsubst {} \;

With sed:

sed -f script.sed | envsubst