How do you check if $* is empty? In other words, how to check if there were no arguments provided to a command?
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Sachin Divekar
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well actually
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3 Answers
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To check if there were no arguments provided to the command, check value of $# variable then,
if [ $# -eq 0 ]; then
>&2 echo "No arguments provided"
exit 1
fi
If you want to use $*(not preferable) then,
if [ "$*" == "" ]; then
>&2 echo "No arguments provided"
exit 1
fi
Some explanation:
The second approach is not preferable because in positional parameter expansion * expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That means a string is constructed. So there is extra overhead.
On the other hand # expands to the number of positional parameters.
Example:
$ command param1 param2
Here,
Value of $# is 2 and value of $* is string "param1 param2" (without quotes), if IFS is unset. Because if IFS is unset, the parameters are separated by spaces
For more details man bash and read topic named Special Parameters
Stephen Kitt
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Sachin Divekar
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6Or `if ! (($#)); ...`, or `if (($# == 0)); ...`, or `if [ $# -eq 0 ]; ...`, or `! (($#)) && ...`, or `(($#)) || ...` – nicerobot Dec 02 '11 at 21:13
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8`[ $# -eq 0 ]` is the most common form IME. There are edge case where `"$#"` can be empty: if there's a single argument which is empty, or if there are several empty arguments and `$IFS` is empty. – Gilles 'SO- stop being evil' Dec 02 '11 at 23:50
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1The `"$*"` expression will also evaluate to `""` if only one `""` parameter was passed. But most of the time you will probably not care about anyway. – manatwork Dec 03 '11 at 13:15
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I believe it should be = instead of == ... currently I'm getting an unexpected operator error, and changing to = fixes the problem. – well actually Dec 05 '11 at 18:02
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@Charlotte That means you are not using `bash`. You are using `sh` to execute the script. AFAIK `==` is valid only in `bash`. – Sachin Divekar Dec 05 '11 at 19:05
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ah, okay. good to know. – well actually Dec 05 '11 at 21:48
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what does `$#` even mean? – Jürgen Paul Sep 28 '14 at 01:50
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@Michelle "# expands to the number of positional parameters." as stated in answer... – MichaelChirico Jun 13 '16 at 16:36
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If you'll indulge me, what is the inverse of this operation? `-neq` didn't work and I'm not doing anything if arguments are missing, so no sense creating that branch unnecessarily. – MichaelChirico Jun 13 '16 at 16:37
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Note, if using getops, make sure to run this if statement check before getops is performed. – Dave Jan 09 '23 at 16:10
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If you're only interested in bailing if a particular argument is missing, Parameter Substitution is great:
#!/bin/bash
# usage-message.sh
: ${1?"Usage: $0 ARGUMENT"}
# Script exits here if command-line parameter absent,
#+ with following error message.
# usage-message.sh: 1: Usage: usage-message.sh ARGUMENT
Pat
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1what is the name of this technique using the `?` in the bracket expansion `${}` ? I cannot wrap myn head around the meaning and behaviour of it. I suppose `?` applies to th `1` in `$1` ; but I'm left clueless, I'd gladly dig deeper this technique/syntax. – Stephane Rolland May 05 '21 at 13:41
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aka parameter expansion: https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html – jgreve Aug 09 '21 at 20:44
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this is one of the ways you can know that you havent got any arguments
NO_ARGS=0
if [ $# -eq "$NO_ARGS" ]; then
{do something}
fi
Joel Dsouza
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8That seems to be a convoluted way of doing what the accepted answer wrote 5 years ago.... – Jeff Schaller Oct 04 '16 at 10:06