Interpreting shell commands within Bash if/else statements

Question

While trying to make a conditional statement to check the amount of files in a directory, I have stumbled upon a problem. My initial way of writing this script:

ELEM=`ls -l $DIR | wc -l` 
if [ $ELEM -lt 5 ] ; then

works. However I want to move my $ELEM into the conditional parameter block so it can be interpreted when I reach that if statement. I have tried playing around with different combinations of single quotes, double quotes, back ticks, and parentheses.

Does anyone know how to solve this problem?

Where is the problem? If makes no difference whether you count your files outside or inside the if statement. — Philipp
– Philipp, Commented Jul 8, 2010 at 14:36

Philipp · Accepted Answer · 2010-07-08 16:07:54Z

2

Never use ls in batch mode, use globbing instead. Also avoid backquotes, unquoted variables, and the [ builtin:

shopt -s nullglob  # expand to an empty array if directory is empty
shopt -s dotglob   # also glob dotfiles
files=("$DIR"/*)
count=${#files[@]}
if ((count > 5))
then
  ...
fi

edited Jul 8, 2010 at 16:07

answered Jul 8, 2010 at 14:18

Philipp

50.1k12 gold badges88 silver badges112 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Melissa Over a year ago

Why is it bad to use 'ls' in in shell scripts? Why do I need to avoid using back ticks, unquoted variables, and brackets? I thought globbing was used primarily for regex manipulations.

Philipp Over a year ago

See mywiki.wooledge.org/BashPitfalls, mywiki.wooledge.org/BashFAQ/082, mywiki.wooledge.org/BashFAQ/031, mywiki.wooledge.org/BashGuide/Patterns, mywiki.wooledge.org/BashGuide/Practices. Bash programming has so many pitfalls that you should really read all pages in Greg's wiki related to Bash before writing any scripts.

Dennis Williamson Over a year ago

@Melissa: Globs are not regular expressions.

Melissa Over a year ago

Heh, I didn't say they were regular expressions.

Philipp Over a year ago

The matter is made complicated because bash supports both globbing and regexes, with superficially similar syntaxes. To get a reliable file count, set the shell options nullglob and dotglob.

|

Melissa · Accepted Answer · 2010-07-08 14:18:12Z

0

For some reason it didn't occur to me until just now to use this:

ELEM="`ls -l $DIR | wc -l`"

Thanks for your time.

answered Jul 8, 2010 at 14:18

Melissa

1

4 Comments

Michael Mrozek Over a year ago

I'm not sure how that solves your problem; the expression is still evaluated at that line, the result is just quoted

Melissa Over a year ago

Is there anyway to fix this problem?

Philipp Over a year ago

Plus that the quoting is wrong ($DIR) is unquoted. Also I don't see the problem at all: it is completely equivalent whether the counting happens as part of an assignment or inside a test.

Melissa Over a year ago

My intention was to completely avoid assigning the variable if I could. Thank you for your help.

Dennis Williamson · Accepted Answer · 2010-07-08 16:03:35Z

0

What you seem to want to be able to do is something that C allows:

if (elem = countfiles(dir) < 5)

Bash can do it like this:

if (( (elem = $(ls "$DIR" 2>/dev/null | wc -l) ) < 5))

Or you could create a function called countfiles and use it like this:

if (( ( elem = $(countfile "$DIR") ) < 5))

where the function might be something like:

countfiles() {
    find "$1" -maxdepth 1 -type f | wc -l
}

answered Jul 8, 2010 at 16:03

Dennis Williamson

364k95 gold badges386 silver badges446 bronze badges

2 Comments

Philipp Over a year ago

The fourth version won't work on file names containing newlines, and the second one will probably be nonportable unless the behavior of ls w.r.t. file names with newlines is standardized. Why calling two external processes for something that Bash can do without external help?

Dennis Williamson Over a year ago

@Philipp: The point of my answer was not to address the issues with using ls inappropriately or other pitfalls of Bash. You covered those well in your comment attached to your answer. I should have made reference to that in my answer for clarity. The point of my answer was to address the evidence that I saw in the OP's question and various comments that there seemed to be a desire to use C-style side effects to set a variable and test it at the same time. Speaking of filenames, this is required reading.

Gordon Davisson · Accepted Answer · 2010-07-08 18:09:18Z

0

Most of the solutions posted so far are (IMHO) overcomplicated. As long as the ls command on your system supports the -B option (display special characters as octal escape sequences), you can just use:

if [[ $(ls -B "$DIR" | wc -l) -lt 5 ]]; then

Note that while parsing ls is generally a bad idea, in this case all we're trying to do is find the number of listings, so there's a lot less to go wrong than usual. In fact, the only thing that wc -l cares about is the number of lines, so the only thing that could go wrong is that a filename might have a newline in it, and hence be mistaken for two filenames; using the -B option to ls protects against this, so it winds up being safe (at least as far as I can see).

answered Jul 8, 2010 at 18:09

Gordon Davisson

128k16 gold badges141 silver badges168 bronze badges

Comments

tilo · Accepted Answer · 2010-07-08 18:29:39Z

0

Improved countfiles function (which doesn't get confused by newlines in file names):

countfiles() {
   find "$1" -maxdepth 1 -type f -print0 | tr -dc '\0' | wc -c
}

answered Jul 8, 2010 at 18:29

tilo

1

Collectives™ on Stack Overflow

Interpreting shell commands within Bash if/else statements

5 Answers 5

8 Comments

4 Comments

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

8 Comments

4 Comments

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related