Checking the number of param in bash shell script

Question

I use for checking the number of params in bash shell as follows:

#! /bin/bash
usage() {
    echo "Usage: $0 <you need to specify at least 1 param>"
    exit -1
}
[ x = x$1 ] && usage

where, if [ x = x$1 ] condition is not satisfied, execute usage. Here, my question is, I never really think about the expression [ x = x$1 ] which looks a lot like a condition expression. Is x counted as a literal? and how come can we use = for comparison. Typically should it be something like ==?

Could anybody please fill the void here?

[ x = x$1 is ] is an archaic trick from a time when one particular shell couldn't handle [ "$1" = "" ] correctly. It is no longer needed; testing against an empty string is safe. — chepner
– chepner, Commented Dec 6, 2013 at 14:09

simryang · Accepted Answer · 2013-12-06 04:39:30Z

1

[ x = y ] condition is for comparing strings. (just once =)

x$1 means concatenating two string x and $1.

So, if $1 is empty, x$1 equals x, and [ x = x ] will be true as a result.

edited Dec 6, 2013 at 4:39

answered Dec 6, 2013 at 4:26

simryang

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2 Comments

jaeyong Over a year ago

I just found out that [ x == y ] and [x = y] are symantically the same.

simryang Over a year ago

tutorialspoint.com/unix/unix-basic-operators.htm I found many examples of '=', but a little one of '==' in string comparison. So, I searched for that when '==' is used, above page says it is used by arithmetic comparison.

janos · Accepted Answer · 2013-12-07 05:49:57Z

0

[ x = x$1 ] is error prone, don't use it. Do this instead [ "$1" ]

The difference is that if $1 contains a space or other special characters, your script will crash, for example:

$ a='hello world'
$ [ x = x$a ] && echo works
-bash: [: too many arguments

To fix this you could do [ x = x"$1" ], but [ "$1" ] is shorter, so what's the point.

[] expressions are used extensively in shell scripts, I recommend to read help test. The [ is a synonym for the "test" builtin, but the last argument must be a literal ], to match the opening [. In there you will find the explanation of the differences between = and == operators.

Finally, literal text in conditions is evaluated to true if not empty. Some more examples:

[ x ]       # true
[ abc ]     # true
[ a = a ]   # true
[ a = x ]   # false
[ '' ]      # false
[ b = '' ]  # false

Also common gotchas are these:

[ 0 ]       # true
[ -n ]      # true
[ -blah ]   # true
[ false ]   # true

These are true, because 0, -n, false or anything being the only argument, they are treated as literal strings, and so the condition evaluates to true.

edited Dec 7, 2013 at 5:49

answered Dec 6, 2013 at 5:22

janos

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1 Comment

jaeyong Over a year ago

It certainly is easier to remember and use

rendon · Accepted Answer · 2013-12-06 04:22:13Z

0

Use $# to count the number of parameters and use the OR operator instead of the AND so that usage() only gets executed if the first condition fails.

#! /bin/bash
usage() {
    echo "Usage: $0 <you need to specify at least 1 param>"
    exit -1
}
[ x = $# ] || usage

answered Dec 6, 2013 at 4:22

rendon

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1 Comment

jaeyong Over a year ago

Oh, maybe I was not clear about my question. My question is more likely, what does [x = x$1 ] mean? and shouldn't it be something like [ x == x$1 ]?

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