help on explaination of shell script

ashley75 · 01-15-2015, 10:15 AM

Hi all,

can someone please explain to me these three line?

filename=`basename $0`

if [[ $# = 1 ]] ; then
LocalHost=$1

what I am not clear is the O and 1.

thanks,

J Martin Rushton · 01-15-2015, 10:23 AM

$0 is the string used to invoke the script, for instance "/home/bin/mytest" or "./test2".

$# is the number of command line arguments, so the quoted example is seeing if there is exactly one command argument and setting $LocalHost to it. The 1 is therefore just a number.

pan64 · 01-15-2015, 12:02 PM

$1 would be the first argument passed to the script, so for example: ./scriptname some words
$0 will be ./scriptname, $1 is some and $2 is words. And $# is 2 in this case.
but there is an error
if [[ $# -eq 1 ]]; then would be correct syntax

rknichols · 01-15-2015, 03:16 PM

Quote:

Originally Posted by pan64

but there is an error
if [[ $# -eq 1 ]]; then would be correct syntax

So, you're saying that comparing the string "1" with the string "1" for equality is insufficient and you need to make the shell convert each string to an integer and compare the two integers, right?

Unless your numbers can potentially have different representations ("1", "01", "0x1", "0x0001", etc.), a string compare for equality is fine, and that is what is commonly done. For greater-than or less-than, not so good, though. The string "10" is less than "9".

cfajohnson · 01-15-2015, 09:08 PM

Quote:

Originally Posted by pan64

$1 would be the first argument passed to the script, so for example: ./scriptname some words
$0 will be ./scriptname, $1 is some and $2 is words. And $# is 2 in this case.
but there is an error
if [[ $# -eq 1 ]]; then would be correct syntax

No, either [[ $# == 1 ]] or [ $# -eq 1 ] or (( $# == 1 )) would be correct.

pan64 · 01-16-2015, 12:19 AM

Quote:

Originally Posted by rknichols

So, you're saying that comparing the string "1" with the string "1" for equality is insufficient and you need to make the shell convert each string to an integer and compare the two integers, right?

No.
I tried to say:
if [[ $# = 1 ]] ; then
is not ok (as in the original post only one = was used). From the other hand using numerical comparison looks better for numbers, but you are right, actually it is completely irrelevant (compare them as number and/or strings is the same right now)

Philip Lacroix · 01-16-2015, 02:38 AM

Quote:

Originally Posted by pan64

No.
I tried to say:
if [[ $# = 1 ]] ; then
is not ok (as in the original post only one = was used).

You can indeed use one single "=" as well. In fact, "=" and "==" are synonymous, at least in bash. But "=" is recommended for POSIX conformance.

Code:

string comparison

=
    is equal to
    if [ "$a" = "$b" ]

==
    is equal to
    if [ "$a" == "$b" ]

    This is a synonym for =

(ABS Guide: Comparison operators)

Code:

string1 == string2
string1 = string2

       True if the strings are equal.  = should be used with the test
       command for POSIX conformance.

(man bash: Conditional Expressions)

pan64 · 01-16-2015, 02:48 AM

ok, thanks, so that is only valid in c.

AnanthaP · 01-16-2015, 03:12 AM

AFAIR, the reasoning is slightly different.

http://www.gnu.org/software/bash/man...al-Expressions

The -eq format is used for numeric comparisons and the = or ==for string comparisons.

OK