Test if a command outputs an empty string

问题:

How can I test if a command outputs an empty string?

回答1:

_{Previously, the question asked how to check whether there are files in a directory. The following code achieves that, but see rsp's answer for a better solution.}

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Empty output

Commands don’t return values – they output them. You can capture this output by using command substitution; e.g. $(ls -A). You can test for a non-empty string in Bash like this:

if [[ $(ls -A) ]]; then
    echo "there are files"
else
    echo "no files found"
fi

Note that I've used -A rather than -a, since it omits the symbolic current (.) and parent (..) directory entries.

Note: As pointed out in the comments, command substitution doesn't capture trailing newlines. Therefore, if the command outputs only newlines, the substitution will capture nothing and the test will return false. While very unlikely, this is possible in the above example, since a single newline is a valid filename! More information in this answer.

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Exit code

If you want to check that the command completed successfully, you can inspect $?, which contains the exit code of the last command (zero for success, non-zero for failure). For example:

files=$(ls -A)
if [[ $? != 0 ]]; then
    echo "Command failed."
elif [[ $files ]]; then
    echo "Files found."
else
    echo "No files found."
fi

More info here.

回答2:

TL;DR

if [[ $(ls -A | head -c1 | wc -c) -ne 0 ]]; then ...; fi

Thanks to netj for a suggestion to improve my original:
if [[ $(ls -A | wc -c) -ne 0 ]]; then ...; fi

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This is an old question but I see at least two things that need some improvement or at least some clarification.

First problem

First problem I see is that most of the examples provided here simply don't work. They use the ls -al and ls -Al commands - both of which output non-empty strings in empty directories. Those examples always report that there are files even when there are none.

For that reason you should use just ls -A - Why would anyone want to use the -l switch which means "use a long listing format" when all you want is test if there is any output or not, anyway?

So most of the answers here are simply incorrect.

Second problem

The second problem is that while some answers work fine (those that don't use ls -al or ls -Al but ls -A instead) they all do something like this:

1. run a command
2. buffer its entire output in RAM
3. convert the output into a huge single-line string
4. compare that string to an empty string

What I would suggest doing instead would be:

1. run a command
2. count the characters in its output without storing them
   • or even better - count the number of maximally 1 character using head -c1
     (thanks to netj for posting this idea in the comments below)
3. compare that number with zero

So for example, instead of:

if [[ $(ls -A) ]]

I would use:

if [[ $(ls -A | wc -c) -ne 0 ]]
# or:
if [[ $(ls -A | head -c1 | wc -c) -ne 0 ]]

Instead of:

if [ -z "$(ls -lA)" ]

I would use:

if [ $(ls -lA | wc -c) -eq 0 ]
# or:
if [ $(ls -lA | head -c1 | wc -c) -eq 0 ]

and so on.

For small outputs it may not be a problem but for larger outputs the difference may be significant:

$ time [ -z "$(seq 1 10000000)" ]

real    0m2.703s
user    0m2.485s
sys 0m0.347s

Compare it with:

$ time [ $(seq 1 10000000 | wc -c) -eq 0 ]

real    0m0.128s
user    0m0.081s
sys 0m0.105s

And even better:

$ time [ $(seq 1 10000000 | head -c1 | wc -c) -eq 0 ]

real    0m0.004s
user    0m0.000s
sys 0m0.007s

Full example

Updated example from the answer by Will Vousden:

if [[ $(ls -A | wc -c) -ne 0 ]]; then
    echo "there are files"
else
    echo "no files found"
fi

Updated again after suggestions by netj:

if [[ $(ls -A | head -c1 | wc -c) -ne 0 ]]; then
    echo "there are files"
else
    echo "no files found"
fi

Discarding whitespace

If the command that you're testing could output some whitespace that you want to treat as an empty string, then instead of:

| wc -c

you could use:

| tr -d ' \n\r\t ' | wc -c

or with head -c1:

| tr -d ' \n\r\t ' | head -c1 | wc -c

or something like that.

Summary

1. First, use a command that works.

2. Second, avoid unnecessary storing in RAM and processing of potentially huge data.

The answer didn't specify that the output is always small so a possibility of large output needs to be considered as well.

回答3:

if [ -z "$(ls -lA)" ]; then
  echo "no files found"
else
  echo "There are files"
fi

This will run the command and check whether the returned output (string) has a zero length. You might want to check the 'test' manual pages for other flags.

Use the "" around the argument that is being checked, otherwise empty results will result in a syntax error as there is no second argument (to check) given!

Note: that ls -la always returns . and .. so using that will not work, see ls manual pages. Furthermore, while this might seem convenient and easy, I suppose it will break easily. Writing a small script/application that returns 0 or 1 depending on the result is much more reliable!

回答4:

For those who want an elegant, bash version-independent solution (in fact should work in other modern shells) and those who love to use one-liners for quick tasks. Here we go!

ls | grep . && echo 'files found' || echo 'files not found'

(note as one of the comments mentioned, ls -al and in fact, just -l and -a will all return something, so in my answer I use simple ls

回答5:

As Jon Lin commented, ls -al will always output (for . and ..). You want ls -Al to avoid these two directories.

You could for example put the output of the command into a shell variable:

v=$(ls -Al)

An older, non-nestable, notation is

v=`ls -Al`

but I prefer the nestable notation $( ... )

The you can test if that variable is non empty

if [ -n "$v" ]; then
    echo there are files
else
    echo no files
fi

And you could combine both as if [ -n "$(ls -Al)" ]; then

回答6:

Bash Reference Manual

6.4 Bash Conditional Expressions

-z string
     True if the length of string is zero.

-n string
string
     True if the length of string is non-zero.

You can use shorthand version:

if [[ $(ls -A) ]]; then
  echo "there are files"
else
  echo "no files found"
fi

回答7:

I'm guessing you want the output of the ls -al command, so in bash, you'd have something like:

LS=`ls -la`

if [ -n "$LS" ]; then
  echo "there are files"
else
  echo "no files found"
fi

回答8:

Here's a solution for more extreme cases:

if [ `command | head -c1 | wc -c` -gt 0 ]; then ...; fi

This will work

• for all Bourne shells;
• if the command output is all zeroes;
• efficiently regardless of output size;

however,

• the command or its subprocesses will be killed once anything is output.

回答9:

Here's an alternative approach that writes the std-out and std-err of some command a temporary file, and then checks to see if that file is empty. A benefit of this approach is that it captures both outputs, and does not use sub-shells or pipes. These latter aspects are important because they can interfere with trapping bash exit handling (e.g. here)

tmpfile=$(mktemp)
some-command  &> "$tmpfile"
if [[ $? != 0 ]]; then
    echo "Command failed"
elif [[ -s "$tmpfile" ]]; then
    echo "Command generated output"
else
    echo "Command has no output"
fi
rm -f "$tmpfile"

回答10:

Sometimes you want to save the output, if it's non-empty, to pass it to another command. If so, you could use something like

list=`grep -l "MY_DESIRED_STRING" *.log `
if [ $? -eq 0 ]
then
    /bin/rm $list
fi

This way, the rm command won't hang if the list is empty.

回答11:

All the answers given so far deal with commands that terminate and output a non-empty string.

Most are broken in the following senses:

• They don't deal properly with commands outputting only newlines;
• starting from Bash≥4.4 most will spam standard error if the command output null bytes (as they use command substitution);
• most will slurp the full output stream, so will wait until the command terminates before answering. Some commands never terminate (try, e.g., yes).

So to fix all these issues, and to answer the following question efficiently,

How can I test if a command outputs an empty string?

you can use:

if read -n1 -d '' < <(command_here); then
    echo "Command outputs something"
else
    echo "Command doesn't output anything"
fi

You may also add some timeout so as to test whether a command outputs a non-empty string within a given time, using read's -t option. E.g., for a 2.5 seconds timeout:

if read -t2.5 -n1 -d '' < <(command_here); then
    echo "Command outputs something"
else
    echo "Command doesn't output anything"
fi

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Remark. If you think you need to determine whether a command outputs a non-empty string, you very likely have an XY problem.