Test if a command outputs an empty string

Question

How can I test if a command outputs an empty string

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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     lt   lt  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     lt   lt  command here   then     echo  Command outputs something  else     echo  Command doesn t output anything  fi     Remark  If you think you need to determine whether a command outputs a non-empty string  you very likely have an XY problem

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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   amp  gt    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

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As mentioned by tripleee in the question comments   use moreutils ifne  if input not empty   In this case we want ifne -n which negates the test  ls -A  tmp empty   ifne -n command-to-run-if-empty-input  The advantage of this over many of the another answers when the output of the initial command is non-empty  ifne will start writing it to STDOUT straight away  rather than buffering the entire output then writing it later  which is important if the initial output is slowly generated or extremely long and  would overflow the maximum length of a shell variable  There are a few utils in moreutils that arguably should be in coreutils -- they re worth checking out if you spend a lot of time living in a shell  In particular interest to the OP may be dirempty exists tool which at the time of writing is still under consideration  and has been for some time  it could probably use a bump

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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

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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  quot  v quot     then     echo there are files else     echo no files fi  And you could combine both as if   -n  quot   ls -Al  quot     then Sometimes  ls may be some shell alias  You might prefer to use    bin ls -Al   See ls 1  and hier 7  and environ 7  and your    bashrc  if your shell is GNU bash  my interactive shell is zsh  defined in  etc passwd - see passwd 5  and chsh 1

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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

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sometimes  quot something quot  may come not to stdout but to the stderr of the testing application  so here is the fix working more universal way  if      partprobe   1  2 gt  amp 1   wc -c  -ne 0     then     echo  quot require fixing GPT parititioning quot  else     echo  quot no GPT fix necessary quot  fi

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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

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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

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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    amp  amp  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

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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     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     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

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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

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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    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    run a command buffer its entire output in RAM convert the output into a huge single-line string compare that string to an empty string   What I would suggest doing instead would be    run a command 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   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   Additional update by jakeonfire   grep will exit with a failure if there is no match  We can take advantage of this to simplify the syntax slightly   if ls -A   head -c1   grep -E      then     echo  there are files  fi  if   ls -A   head -c1   grep -E      then     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   First  use a command that works  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

[bash] Test if a command outputs an empty string

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