SC2148: Missing Shebang Line in Shell Scripts

The Shebang Line

The shebang (hash-bang) is the first line of a script that tells the operating system which interpreter to use. Without it, the behavior depends on the calling shell and is unpredictable.

Common Shebangs

#!/bin/bash          # Bash (absolute path)
#!/usr/bin/env bash  # Bash (portable, uses PATH lookup)
#!/bin/sh            # POSIX shell (may be dash, ash, etc.)
#!/usr/bin/env sh    # POSIX shell (portable)
#!/bin/zsh           # Zsh

Why It Matters

Without a shebang, executing ./script.sh may use:

• The current shell as interpreter
• /bin/sh on some systems
• Fail entirely on others

# Without shebang, this Bash-specific syntax might fail:
if [[ $var =~ regex ]]; then
  echo "match"
fi
# If executed by /bin/sh (dash), [[ is a syntax error

#!/bin/bash vs #!/usr/bin/env bash

Approach	Pros	Cons
#!/bin/bash	Direct, fast	Fails if bash is not at /bin/bash (NixOS, FreeBSD)
#!/usr/bin/env bash	Portable, finds bash in PATH	Slightly slower, cannot pass flags on some systems

#!/bin/sh vs #!/bin/bash

Using #!/bin/sh means your script must be POSIX-compliant. Bash-specific features are not available:

Feature	POSIX sh	Bash
[[ ]]	No	Yes
Arrays	No	Yes
${var//pattern/replace}	No	Yes
function keyword	No	Yes
Process substitution <()	No	Yes

Best Practice

Use #!/usr/bin/env bash for scripts that use Bash features, and #!/bin/sh only when you have verified POSIX compliance.