Bash脚本创建配置文件报错Bad Substitution的原因与解决方法

错误信息 #

line 2: filebeat.inputs:
- type: filestream
  id: my-filestream-id
  enabled: true
: bad substitution

问题脚本 #

#!/bin/bash
cat > "config.yml" <<EOF
filebeat.inputs:
- type: filestream
  id: my-filestream-id
  enabled: true
  paths:
    - /home/ubuntu/logs/**/*.log
filebeat.config.modules:
  # Glob pattern for configuration loading
  path: ${path.config}/modules.d/*.yml
  reload.enabled: false
setup.template.settings:
  index.number_of_shards: 1
setup.kibana:
output.logstash:
  hosts: ["10.09.0.2:5044"]
processors:
  - add_host_metadata:
      when.not.contains.tags: forwarded
  - add_cloud_metadata: ~
  - add_docker_metadata: ~
  - add_kubernetes_metadata: ~
EOF

错误原因 #

Bash会自动解析<<EOF格式here-doc里的变量，而你写的${path.config}不符合Bash变量命名规则（变量名仅允许字母、数字、下划线，不能包含.），导致Bash无法识别该语法，抛出"bad substitution"错误。

解决方法 #

有两种简单修复方式：

方法1：用单引号包裹EOF，关闭变量解析 #

将脚本中的<<EOF改为<<'EOF'，这样Bash会原样输出here-doc内的所有内容，不再解析任何变量：

#!/bin/bash
cat > "config.yml" <<'EOF'
filebeat.inputs:
- type: filestream
  id: my-filestream-id
  enabled: true
  paths:
    - /home/ubuntu/logs/**/*.log
filebeat.config.modules:
  # Glob pattern for configuration loading
  path: ${path.config}/modules.d/*.yml
  reload.enabled: false
setup.template.settings:
  index.number_of_shards: 1
setup.kibana:
output.logstash:
  hosts: ["10.09.0.2:5044"]
processors:
  - add_host_metadata:
      when.not.contains.tags: forwarded
  - add_cloud_metadata: ~
  - add_docker_metadata: ~
  - add_kubernetes_metadata: ~
EOF

方法2：转义变量符号，让Bash忽略解析 #

把${path.config}修改为\${path.config}，通过反斜杠转义$符号，让Bash将其视为普通字符串输出：

#!/bin/bash
cat > "config.yml" <<EOF
filebeat.inputs:
- type: filestream
  id: my-filestream-id
  enabled: true
  paths:
    - /home/ubuntu/logs/**/*.log
filebeat.config.modules:
  # Glob pattern for configuration loading
  path: \${path.config}/modules.d/*.yml
  reload.enabled: false
setup.template.settings:
  index.number_of_shards: 1
setup.kibana:
output.logstash:
  hosts: ["10.09.0.2:5044"]
processors:
  - add_host_metadata:
      when.not.contains.tags: forwarded
  - add_cloud_metadata: ~
  - add_docker_metadata: ~
  - add_kubernetes_metadata: ~
EOF

内容的提问来源于stack exchange，提问作者eme