If script.sh is just something typical like
#!/bin/bash echo "Hello World!"
Is there a preferred way to run the script? I think you first have to chmod it so it becomes executable?
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For your specific script either way will work, except that
./script.sh requires execution and readable bits, while
bash script.sh only requires readable bit.
The reason of the permissions requirement difference lies in how the program that interprets your script is loaded:
./script.shmakes your shell run the file as if it was a regular executable.
The shell forks itself and uses a system call (e.g.
execve) to make the operating system execute the file in the forked process. The operating system will check the file's permissions (hence the execution bit needs to be set) and forward the request to the program loader, which looks at the file and determines how to execute it. In Linux compiled executables start with an ELF magic number, while scripts start with a
#! (hashbang). A hashbang header means that the file is a script and needs to be interpreted by the program that is specified after the hashbang. This allows a script itself to tell the system how to interpret the script.
With your script, the program loader will execute
/bin/bash and pass
./script.sh as the command-line argument.
bash script.shmakes your shell run
script.shas the command-line argument
So the operating system will load
bash (not even looking at
script.sh, because it's just a command-line argument). The created
bash process will then interpret the
script.sh because it's passed as the command-line argument. Because
script.sh is only read by
bash as a regular file, the execution bit is not required.
I recommend using
./script.sh though, because you might not know which interpreter the script is requiring. So let the program loader determine that for you.
bash script.sh invokes the script directly using the bash.
./script.sh is using the shebang
#!/bin/bash to determine how to execute.
If you really want to know, which binary is executed if you do a
bash script.sh you could find out with
So in your example it makes no difference. Yes, you have to
chmod +x script.sh to be able to execute it directly via
Create a file Delete_Self.sh like this:
#!/bin/rm echo I am still here!
Run this script as
sh Delete_Self.sh you will see "I am still here!" echoed back.
Make it executable, and run it as
./Delete_Self.sh you will see nothing is echoed back, while the file
Delete_Self.sh itself is gone.
So the difference is that:
bash script.shwill ignore the #! line, because bash is specified as the program to run script.sh.
./script.shwill read the #! line to determine the program to run
In addition to the other answers, knowing the difference between running a script via
./script.sh (i) and source
./script.sh (ii) is useful - The (i) version creates a new shell in which to run the command, whereas (ii) runs it in the current shell - which can be mandatory if the executable changes environment variables that need to be preserved after the executable exits. For example, to activate a python conda environment the following must be used:
source activate my_env
N.B. Another alternative to
source that you may encounter is the
. builtin, i.e.
. activate my_env