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When an attempt is made to execute a file, the kernel will check the file [[Linux Permissions|permissions]]. If the effective user has permission to execute the file, it will be opened and the first few bytes examined for a 'magic number', which will identify the type of executable. If no magic number is found, and the file is a text file, it will be interpreted by the current shell (identified by the SHELL environment variable).
If the first two characters of the file are "#!" (which is considered a magic number), then the rest of the first line of the file is used as the absolute path and arguments for the interpreter. This is called a "shbang" line, from "#" (sharp) and "!" (bang). Therefore, using this as the first line of your [[BASH]] script:
#!/bin/bash
Will force the kernel to launch /bin/bash as the interpreter for the script. This ensures that bash will used even if the user prefers ksh, csh, or some other shell for their command-line work.
There are three requirements for a valid bash [[BASH]] script:
# The first line of the script must be <code>#!/bin/bash</code>
# The rest of the file must contain bash [[BASH]] commands.
# The file must be made executable (see [[Linux Permissions]]).