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Why exactly is bash <script different than bash script? When i try this using a script that contains:

cat
hello
world

In the first case, i get what i expected:

hello
world

But in the second case, i need to type a ctrl-D and then i get:

script: line 2: hello: command not found
script: line 3: world: command not found

I assume this means in the second case bash does not read line 2 and 3 as input for cat but as instructions instead.

I thought bash arguments are handled simply but opening the corresponding file and using dup2() to set that script file to the file descriptor 0, so having a file as argument or as an input redirection shouldn't be too different.

Is bash instead reading the entire file and then executing it line by line?

1 Answer 1

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In the first case, feeding the script to bash as stdin:

  • bash runs cat
  • cat reads the rest of the file from stdin and copies it to stdout

In the second case:

  • bash runs the cat command
  • cat reads from stdin (which isn't the file) until you press ctrl-d to close cat
  • bash reads and runs the next line...

Note that when you put a filename on the command line like this, typically the file is a bash script and considered a program itself.

An alternate way to do this would be to put this in your file:

cat <<EOF
hello
world
EOF

In this case, stdin is redirected by the shell and cat is fed the lines up to the second EOF. This is sometimes called a HERE document. You can replace EOF (in both places) with whatever marker you want.

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