Silent Bash Substitution – How to Make Bash Substitution Silent

bashcommand-substitutionstderr

I want to replace cat:

var=$(cat "filename" 2>/dev/null)

by bashism syntax:

var=$(<"filename")

The problem is that I don't know how to make the bashism silent to avoid such warnings:

bash: filename: No such file or directory

I've tried this:

var=$(2>/dev/null <"filename")

but it does not read existing files into var anymore.

Best Answer

Wrapping the assignment into a compound block and using a redirection on that seems to work:

{ var=$(<"$file"); } 2>/dev/null;

e.g.

$ echo hello > test1; rm -f test2
$ file=test1; { var=$(<"$file"); } 2>/dev/null; echo "${var:-[it is empty]}"
hello
$ file=test2; { var=$(<"$file"); } 2>/dev/null; echo "${var:-"[it is empty]"}"
[it is empty]

Just don't use ( .. ) to create a subshell, since then the assigned variable would be lost.

Example

Say I have the following sample directory.

$ tree
.
|-- dir1
|   `-- a\ file1.txt
|-- dir2
|   `-- a\ file2.txt
|-- dir3
|   `-- a\ file3.txt
`-- myscript

3 directories, 4 files

Now let's say I have this for ./myscript.

#!/bin/bash

for i in "$@"; do
    echo "file: $i"
done

Now when I run the following command.

$ find . -type f -print0 | xargs -r0 ./myscript 
file: ./dir2/a file2.txt
file: ./dir3/a file3.txt
file: ./dir1/a file1.txt
file: ./myscript

Or when I use the 2nd form like so:

$ find . -type f -exec ./myscript {} +
file: ./dir2/a file2.txt
file: ./dir3/a file3.txt
file: ./dir1/a file1.txt
file: ./myscript

Details

find + xargs

The above 2 methods, though looking different, are essentially the same. The first is taking the output from find, splitting it using NULLs (\0) via the -print0 switch to find. The xargs -0 is specifically designed to take input that's split using NULLs. That non-standard syntax was introduced by GNU find and xargs but is also found nowadays in a few others like most recent BSDs. The -r option is required to avoid calling myscript if find finds nothing with GNU find but not with BSDs.

NOTE: This entire approach hinges on the fact that you'll never pass a string that's exceedingly long. If it is, then a 2nd invocation of ./myscript will get kicked off with the remainder of subsequent results from find.

find with +

That's the standard way (though it was only added relatively recently (2005) to the GNU implementation of find). The ability to do what we're doing with xargs is literally built into find. So find will find a list of files and then pass that list as as many arguments as can fit to the command specified after -exec (note that {} can only be last just before + in this case), running the commands several times if needed.

Why no quoting?

In the first example we're taking a shortcut by completely avoiding the issues with the quoting, by using NULLs to separate the arguments. When xargs is given this list it's instructed to split on the NULLs effectively protecting our individual command atoms.

In the second example we're keeping the results internal to find and so it knows what each file atom is, and will guarantee to handle them appropriately, thereby avoiding the whoie business of quoting them.

Maximum size of command line?

This question comes up from time to time so as a bonus I'm adding it to this answer, mainly so I can find it in the future. You can use xargs to see what the environment's limit like so:

$ xargs --show-limits
Your environment variables take up 4791 bytes
POSIX upper limit on argument length (this system): 2090313
POSIX smallest allowable upper limit on argument length (all systems): 4096
Maximum length of command we could actually use: 2085522
Size of command buffer we are actually using: 131072

Best Answer

Related Solutions

Bash and Tab Auto-complete

Bash – Using a generated list of filenames as argument list — with spaces

Example

Details

Why no quoting?

Maximum size of command line?

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