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I have a file containing a list of directories. For instance


I want to create all these directories. Here's what I did:

for dir in $(cat myfile); do mkdir $dir; done

What would be the correct way of doing this while avoiding the "useless use of cat"?

Ideally, answers would focus on Ksh88, but I'm also interested in other shells

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if all the directories are under single directory and are few then use brace expansion of shell. $ mkdir /foo/bar/dir{1,2,3} or $ mkdir /foo/bar/{dir1,dir2,dir3} –  Nikhil Mulley Dec 19 '11 at 19:39
possible duplicate of How can I read line by line from a variable in bash? –  Gilles Dec 19 '11 at 23:33
Thanks Gilles for pointing out that question, I was not aware of it. For the record, I was looking to read from a file not a variable. And my question is shell-agnostic (if anything I mention ksh). In any case I found my answer; I was not familiar with read. The man page will do the rest. –  rahmu Dec 20 '11 at 13:54
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5 Answers

up vote 12 down vote accepted

AProgrammer's suggestion of using xargs is often best, but another option is to use redirection into a while loop, which allows additional commands to be made and variables to be set:

while read -r dir; do mkdir $dir; done < myfile

An example of a more complicated structure would be:

now=`date +%Y%m%d.%H%M%S`
while read -r dir; do
    newdistfile="/tmp/dist-`echo $dir | tr / _`.tgz"
    mv $dir ~/backups/$dir.$now &&
        mkdir $dir &&
        tar xzfC $newdistfile $dir
done < myfile

This is not something that xargs could do without writing a 'helper program'.

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What are the limits of using xargs over read? It is my understanding that xargs will fork a new process (unlike read that is a shell builtin). Isn't this something you want to avoid? –  rahmu Dec 19 '11 at 16:40
xargs will operate on a single command, so if you want to have a compound statement (like more than one, an if, while or for, etc.) or setting a variable, then you cannot use xargs. Yes, xargs will fork one new statement, plus one for some number of input lines, which is very little overhead. However, some older versions of xargs do not take a -0 option, so a while read will be the way to go if there may be whitespace in the input directory names. –  Arcege Dec 19 '11 at 17:38
Thank you for your clear explanations! –  rahmu Dec 20 '11 at 13:53
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I'd do something like

xargs mkdir < myfile
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Why "-n 1"? You can do several directories with one invocation of mkdir. –  ddeimeke Dec 19 '11 at 16:29
@ddeimeke, because I didn't remember if it is POSIX or just a common extension. Now I've checked and it is in SUSv2. –  AProgrammer Dec 19 '11 at 16:39
Useless use of xargs. ;) –  user unknown Dec 19 '11 at 19:25
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At least in bash, as long as there are no filenames containing spaces and newlines, this:

mkdir $(< myfile) 

works. So we have a useless use of for, xargs too.

< does not start a new process in bash, in contrast to cat, but I don't know for ksh.

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+1 for clarifying, as opposed to other answer, the spaces problem. –  enzotib Dec 19 '11 at 20:05
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This is the solution closer to your original script still avoiding the useless cat

for dir in $(<myfile); do mkdir $dir; done

It still contains a useless loop though.

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I wouldn't say that's a useless use of cat.

In the classic sense, a UUOC means writing:

cat file | some_command and its args ...

instead of the equivalent and cheaper

<file some_command and its args ...

or (equivalently and more classically)

some_command and its args ... < file

In this case:

for dir in $(cat myfile); do mkdir $dir; done

there is no command into which the output of cat is being piped.

There are certainly alternatives that don't use cat. In particular, for bash and ksh $(<myfile) is likely to be faster than $(cat myfile), though not all shells support that construct. But I wouldn't call the original code a "useless use of cat" any more than I'd call AProgrammer's solution a "useless use of xargs".

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