How to tell "cat" command to not hang on when there is no path provided?

Question

I am simply trying to get a bash script
to return the content of a text file.

#!/bin/bash
cat $1

the problem is.. when the path to a file is not fed to this bash script... how will it even close itself.

because via regular command line... if there is no file path being fed to the cat command.. it does not hang up.

almost as if it is waiting for something..

Answer 1

#!/bin/bash
if [[ "$#" -ne 1 ]]; then
    echo "Usage: $0 [INPUT FILE]" 1>&2
    exit 1
fi
cat "$1"

Answer 2

Your command also has another problem, what if the filename has spaces?

#!/bin/bash
cat "$1"

Always quote unless you have a compelling reason not to. And cat "" does not hang. Instead it would produce an error message like "No such file or directory".

Answer 3

cat will ignore STDIN if you supply it with a filename. Connecting STDIN to /dev/null would be a valid solution in this context.

$ cat test.sh
#!/bin/bash
cat $1 < /dev/null
$ ./test.sh test.sh
#!/bin/bash
cat $1 < /dev/null
$ ./test.sh
$

The shortcoming of this approach is that it will return 0 if no file name is supplied. No way around that. Failure to open the specified file will still return failure.

Answer 4

I would say that cat is an unsuitable tool here and this is why you are getting this undesired behaviour, cat is designed to concatenate a set of files and write the output to another file, it is a quirk of the implementation that the default input is stdin and the default output is stdout.

I would say a far more suitable command here would be:

#!/bin/bash
cp ${1} /dev/stdout

The dev stdout directory is originally Linux only, however before I am yelled at for using a platform specific construct bash (the shell the op explicitly stated they are using) implements /dev/stdout as a builtin so is therefore usable on any platform with bash or ksh.

cp is a utility more suitable here as it is designed to copy the contents from one file to another and that is exactly what we are doing here, copying $1 to stdout

This command will also usefully fail given no input and works on any system with bash, note it does assume that the user has properly escaped any file names with odd chars like space or \n.

Answer 5

To display file contents only if the file exists:

#!/bin/bash
[ -f "$1" ] && cat "$1"

This script assumes that the first argument is the file you want to cat

Answer 6

Default $1 to /dev/null if it does not exist.

(You can easily figure out the bash syntax for that)

Answer 7

: | cat $1

This would be the shortest piece of code that would work. It should ignore the absence of a file silently.

Howeverm, is this really what you need? I agree with @Vality that you probably should not use cat in this context.