9.20 Too Many Files for the Command Line

A pair of backquotes (`` ) (9.16 ) lets you run a command like find (17.1 ) and put its output onto another command's command line. For example:

pr
 lpr
 

% pr 

-n `find . -type f -mtime -1 -print` | lpr

would give a list of all the files you edited today to pr and pipe pr 's output to the printer.

One day I was making global substitutions to a lot of files (34.3 , 28.9 ) and got the error Arguments too long when I tried to print the files I had edited. Turned out that find output such a long list of files that it overflowed the command line:

% pr -n ./

path

/

file1 

./

path

/

path

/

file2 

./

path

/

file3

 ... | lpr

(This can happen for any command inside backquotes, not just find .)

I had to split find 's standard output into chunks that wouldn't be too long. This was on a UNIX that didn't have the great xargs (9.21 ) , so I decided to use fmt (35.2 ) instead. fmt reads its standard input and collects enough text to fill an output line. I used fmt -1000 , which makes output lines about 1000 characters wide-long enough so I wouldn't need too many pr commands, but not too long... I started a Bourne shell, which lets you pipe to the input of a loop (45.23 ) . The shell prints secondary prompts (9.13 ) until you finish entering the loop:

read
 


exit
 

% sh


$ find . -type f -mtime -1 -print |


> fmt -1000 |


> while 

read 

files


> do pr -n $files


> done | lpr


$ exit


%

The shell put each line of filenames from fmt -1000 into the files shell variable, ran pr with those filenames, and piped the output of all the pr s to the standard input of lpr . The lpr command didn't know that it was being fed by lots of pr commands - all it saw was a series of 66 - line pages that the pr s output.

If you have xargs on your system, you can do the same thing this way:

% find . -type f -mtime -1 -print | xargs pr -n | lpr

Parting shot (by ML): there's really no excuse for xargs or any of these other tricks; they're just a patch for a design error. UNIX should be able to handle arbitrarily long command lines; maybe in some future version, it will.