[SOLVED] bash if statement, cannot get the else

MadeInGermany · 07-09-2018, 01:53 AM

Aaarg this hurts

-eq is for comparing numbers!
@Shadow_7, do you confuse it with perl where eq compares strings?

pan64 · 07-09-2018, 07:56 AM

probably irrelevant, but there is a missing " in line 2:

Code:

   echo "line is $line

you need to add a closing " (if not yet made)

Shadow_7 · 07-11-2018, 04:39 PM

Quote:

Originally Posted by MadeInGermany

Aaarg this hurts

-eq is for comparing numbers!
@Shadow_7, do you confuse it with perl where eq compares strings?

Not confused with perl, as I never really did much in perl. Using so many symbols to type your variables seemed inefficient. But I could be confused with a number of other programming options.

$ if [ "YES" -eq "NO" ]; then echo "true"; fi
bash: [: YES: integer expression expected

Seems like it is for "integer expression"'s.

$ if [ "YES" = "YES" ]; then echo "true"; else echo "false"; fi
true

$ if [ "YES" = "NO" ]; then echo "true"; else echo "false"; fi
false

I used to always get messed up in C because using an = in there SETS THE VALUE, and == is the COMPARE operator. Where setting the value will ALWAYS return true. Baring rare exception, solar flares, faulty hardware, ... The always false option would likely not compile.

$ if [ "YES" == "YES" ]; then echo "true"; else echo "false"; fi
true

$ if [ "YES" == "NO" ]; then echo "true"; else echo "false"; fi
false

In bash I don't think it matters as both seem to work.

pan64 · 07-12-2018, 12:21 AM

Quote:

Originally Posted by Shadow_7

N

I used to always get messed up in C because using an = in there SETS THE VALUE, and == is the COMPARE operator.

In bash you can use == for comparison, that will make the life easier (although = and == are exactly the same - as far as I know).
In bash use strings (like -eq, -gt ...) for numerical comparison and use signs (like =, > ..) for string comparison.

MadeInGermany · 07-12-2018, 02:08 AM

Yes, bash treats == like = in the test or [ command.
Because it is a command, you must escape < and > string comparison operators!
See also this

Perhaps you should get used to [[ ]] that is not a command:
== is preferred over =
< and > must not be escaped
$variable does not need quotes because no argument expansion takes place
And it has got a feature:
== treats the right operand as a glob pattern, unless it is quoted.
For demonstration:

Code:

x=55 y=?*
[[ $x == $y ]] && echo match
[[ $x == "$y" ]] && echo equal

The latter is a plain string comparison, as can be done with the [ test command with

Code:

[ "$x" = "$y" ] && echo equal

where the quoting is done for a different reason (and on both sides).

pan64 · 07-12-2018, 03:42 AM

[ can be a bash built-in and/or a binary: /usr/bin/[
test is the same as /usr/bin/[
[[ is only a built-in command

MadeInGermany · 07-12-2018, 08:43 AM

In fact on most Unix OS (but not GNU/Linux) there is a link /usr/bin/[ --> /usr/bin/test
And when /usr/bin/test is invoked as [ it just requires a closing ] as the last argument.

In contrast, the [[ ]] is a built-in, and [[ is not a command.
Within [[ ]] the shell uses a different parser that avoids the problems with the command parser.

The only drawback with [[ ]] is that it is not yet standardized (Posix).
It works in bash/ksh/zsh but maybe not in /bin/sh.

bkelly · 07-12-2018, 04:07 PM

I am new at Linux and Bash. I think I have gotten off track and I don’t understand most of what has been posted. I do not understand the post of GazL at all. MadeInGermany writes about “case” then the code does not look like case at all. Well, not what I am accustomed do.
The first part of my bash script builds a file that is a list of files to look for. In there I decided to put some comments and create them with the # as the first character. That seems to be in keeping with the Linux scripting concept. Now I want the IF statement to skip over them.
The second part does some greps and has that noted if statement. Right now that segment of code looks like this:

Code:

echo “4 line is $LINE >> $LIST_OF_REFERENCES
if [[ “$line” == “#*” ]];
then
    echo “hash found”
else
    echo “hash not found”
fi

The echo statement does output a line beginning with the 4 character and the contents of each line.
The first three lines of the file have the character # as the first character. The first part of the if is never taken. I don't understand why not.

MadeInGermany · 07-12-2018, 04:32 PM

Agreed we went a bit off with an experts discussion.
Still you should be able to get your bits out of the many posts.
Here is the annotated correction for your initial post:

Code:

while IFS= read -r line; do
   echo "line is $line"
# The " at the end was missing!
   if [[ "$line" != "#"* ]]; then
# The * is a glob match (or wildcard) character
# Only [[ ]] provides safe glob matches
# The literal # must be escaped: \# or "#" or '#'
      echo "# not found"
   else
      echo "# was found"
   fi
done

And your previous post:

Code:

if [[ "$line" == "#"* ]]
# The literal # must be escaped but not the * (unless you want to compare with a literal *)
then
    echo "hash found"
else
    echo "hash not found"
fi

scasey · 07-12-2018, 04:33 PM

Yes, there has been a lot of discussion and opportunities for learning here. It's been a most enlightening thread, IMO.

Code:

echo “4 line is $LINE >> $LIST_OF_REFERENCES
if [[ “$line” == “#*” ]];
then
    echo “hash found”
else
    echo “hash not found”
fi

Couple of obvious things. First, is the variable $LINE or $line? ...they are not the same. Case matters.
Use the correct variable name in the if statement.

Re-read the posts and find the one that tells you how to write the regexp for the match.
You are currently trying to match a line that is equal to "#*" That is, one that contains only a "shesplat".

[Oh...MadeInGermany reposted it for you...]

Did you ever put set -x in the script so you could see what it is doing?

[Part of the reason we've been rambling is your 6-day absence...more frequent feedback from you would be helpful, IMO]

bkelly · 07-12-2018, 05:44 PM

scasey,
Sorry about the delay, yes, that makes it a bit difficult. I went through a bunch of iterations today and the working code, only the essential bits, is:

Code:

if [[ "$line" == "#"* ]];
then
   echo "found"
else
   echo "not found"
fi

The # character is not escaped. Then I simplified the IF to

Code:

if [ "$line" == "#"* ];

using only single brackets and it did not work.
OK, so that is the way it is.
I thank each of you for your time and patience with me.

Quite a while ago I coined this phrase and told my boss. I still use it:

The hard stuff?
I do that every day.
Its the easy stuff that kicks my ass!

bkelly · 07-12-2018, 06:06 PM

And I just discovered something surprising. This works:

Code:

if [[ "$line" == "#"* ]];
then
   echo "found"
else
   echo "not found"
fi

but this does not work

Code:

if [[ "$line" == "#"* ]];
then
#   echo "found"
else
   echo "not found"
fi

If seems the IF cannot take a null set of commands. That's not very nice.

keefaz · 07-12-2018, 06:24 PM

For this type of stuff, I prefer filter comments before the read lines loop

Code:

while read -r line; do
  # do any job with $line
  echo $line
done < <(grep -v ^# /path/to/file)

edit, same as grail posted in previous page

scasey · 07-12-2018, 06:49 PM

Quote:

Originally Posted by bkelly

scasey,
Sorry about the delay, yes, that makes it a bit difficult. I went through a bunch of iterations today and the working code, only the essential bits, is:

Code:

if [[ "$line" == "#"* ]];
then
   echo "found"
else
   echo "not found"
fi

The # character is not escaped. Then I simplified the IF to

Code:

if [ "$line" == "#"* ];

using only single brackets and it did not work.
OK, so that is the way it is.
I thank each of you for your time and patience with me.

Quite a while ago I coined this phrase and told my boss. I still use it:

The hard stuff?
I do that every day.
Its the easy stuff that kicks my ass!

MadeInGermany explained why that is in #14

scasey · 07-12-2018, 06:51 PM

Quote:

Originally Posted by bkelly

And I just discovered something surprising. This works:

Code:

if [[ "$line" == "#"* ]];
then
   echo "found"
else
   echo "not found"
fi

but this does not work

Code:

if [[ "$line" == "#"* ]];
then
#   echo "found"
else
   echo "not found"
fi

If seems the IF cannot take a null set of commands. That's not very nice.

Your second snippet simply says to do nothing if the test matches. Why do you think that doesn't work?