GGistDev

Templating and Substitution

Fill templates and generate files using parameter expansion and simple tools.

Parameter expansion patterns

name=${name:-guest}      # default (no assignment)
name=${name:=guest}      # assign default
path=${path%/}           # trim shortest suffix
path=${path%%/*}         # trim longest suffix pattern
msg=${msg/foo/bar}       # replace first
msg=${msg//foo/bar}      # replace all

Here-doc templates

cat > config.ini <<EOF
[app]
name=${APP_NAME}
port=${PORT:-3000}
EOF

Use single-quoted delimiter (<<'EOF') to disable expansion when you want a literal template.

envsubst

export APP_NAME=myapp PORT=3000
envsubst < config.tmpl > config

Substitute ${VAR} placeholders from the environment.

printf and small DSLs

printf '[%s] %s\n' "$level" "$msg" >> app.log

`

Safety and quoting

  • Always quote expansions to avoid word splitting/globbing
  • Validate inputs before substitution to avoid injection into config files

Summary

  • Use parameter expansion and here-docs for simple templating
  • Reach for envsubst for environment-driven configs
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