Command that prints file contents given filename on stdin

catstdinxargs

I have a command that produces some output part of which is actually a file name containing the full output data. I want to extract the file name (which I trivially do with cut) and then pass it to another program such as cat which will display the contents of that file.

I can't share the actual command I am running but for the purposes of reproducibility you can replace ./my-command args with echo 1:2:3:4:file.txt

So I tried the following:

./my-command args | cut -d : -f 5 | cat

However this just prints the file name since cat merely copies the contents of stdin (which contains my filename) to stdout when it is invoked without any arguments.

Now I know that I can work around this is Bash by inverting the flow of commands and essentially passing cat the argument via a sub-shell e.g.

cat `./my-command args | cut -d : -f 5`

And this will print the contents of the file however this is not particularly friendly since I can't simply pipe the output of my underlying command by appending a simple command to my invocation.

Is there a cat like program which will take in filenames on stdin and print their contents? Or is there a way to get cat to treat stdin as arguments?

Notes

I realise that I could define a bash function that would read filenames from stdin and invoke cat on them in my own environment. However this technique for dumping the output data directly needs to be included in end user documentation and a simple shell agnostic approach is highly preferable.

Best Answer

After doing a bit more research I realised that this scenario is exactly what xargs is designed for:

./my-command args | cut -d : -f 5 | xargs cat

Which will transform the output of stdin into an invocation of cat with an actual filename and thus print out the file contents

Related Solutions

Bash Scripting – Use Filename Arguments or Default to stdin/stdout

I'd make heavier use of I/O redirection:

#!/bin/bash
[[ $1 ]] && [[ ! -f $1 ]] && echo "file $1 dne" && exit 1
[[ $1 ]] && exec 3<$1 || exec 3<&0
[[ $2 ]] && exec 4>$2 || exec 4>&1
fgrep -v "stuff" <&3 >&4

Explanation

[[ $1 ]] && [[ ! -f $1 ]] && echo "file $1 dne" && exit 1

Test if an input file has been specified as a command line argument and if the file exists.
[[ $1 ]] && exec 3<$1 || exec 3<&0

If $1 is set, i.e. an input file has been specified, the specified file is opened at file descriptor 3, otherwise stdin is duplicated at file descriptor 3.
[[ $2 ]] && exec 4>$2 || exec 4>&1

Similarly if the $2 is set, i.e. an output file has been specified, the specified file is opened at file descriptor 4, otherwise stdout is duplicated at file descriptor 4.
fgrep -v "stuff" <&3 >&4

Lastly fgrep is invoked, redirecting its stdin and stdout to the previously set file descriptors 3 and 4 respectively.

Reopening standard input and output

If you'd prefer not to open intermediate file descriptors, an alternative is to replace the file descriptors corresponding to stdin and stdout directly with the specified input and output files:

#!/bin/bash
[[ $1 ]] && [[ ! -f $1 ]] && echo "file $1 dne" && exit 1
[[ $1 ]] && exec 0<$1
[[ $2 ]] && exec 1>$2
fgrep -v "stuff"

A drawback with this approach is that you loose the ability to differentiate output from the script itself from the output of the command which is the target for the redirection. In the original approach, you can direct script output to the unmodified stdin and stdout, which in turn might have been redirected by the caller of the script. The specified input and output files could still be accessed via the corresponding file descriptors, which are distinct from the script stdin and stdout.

Does cat never read from terminal

Isn't cat reading from the stdin and stores that that into file "filename"?

Yes, when cat does not have any filename arguments (or if one of the files is the minus character -), it reads from stdin.

Perhaps use of the word "never" by the book is a bit misleading, because:

Is the above excerpt from the book just saying that only the particular form of using cat with a FILE argument never reads from the stdin?

Yes, in that particular instance, cat will not touch stdin.

Notes

Best Answer

Related Solutions

Bash Scripting – Use Filename Arguments or Default to stdin/stdout

Does cat never read from terminal

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