[SOLVED] Simple bash string manipulation

GPGAgent · 11-12-2020, 01:14 PM

I have a file name, abcdefgh.mp4 and I want to just get the last four characters of the name "efgh" (okay - I know files in linux don't have extensions like in Windoze it's alll just part of the name)

I can do it with an intermediate variable, but I want one neat bit of bash:

Code:

$ echo $ff  ${ff%.*} 
5f391985dc5a423cb7d31.mp4 5f391985dc5a423cb7d31

So far so good and I could do this:

Code:

$ X=${ff%.*} 
$ echo ${X: -4}
7d31

Or this, but it presumes the filenames are always the same length:

Code:

~$ ff=5f391985dc5a423cb7d31.mp4
~$ echo ${ff:17:4}
7d31

But I would like a one liner please

individual · 11-12-2020, 01:27 PM

Accounting for the extension in this case, you could use the string length - the extension length (4 including the .).

Code:

ff='5f391985dc5a423cb7d31.mp4'
echo "${ff:$((${#ff} - 8)):4}"

computersavvy · 11-12-2020, 01:36 PM

Code:

 X=`basename -s .mp4 12345dr61.mp4`; echo ${X: -4}

would give you exactly what you asked for in one line.

GPGAgent · 11-12-2020, 02:35 PM

Quote:

Originally Posted by computersavvy

Code:

 X=`basename -s .mp4 12345dr61.mp4`; echo ${X: -4}

would give you exactly what you asked for in one line.

Cheers, I should have been more explicit, I meant one command with no intermediate variable

GPGAgent · 11-12-2020, 02:37 PM

Quote:

Originally Posted by individual

Accounting for the extension in this case, you could use the string length - the extension length (4 including the .).

Code:

ff='5f391985dc5a423cb7d31.mp4'
echo "${ff:$((${#ff} - 8)):4}"

Neat, I'll use it thanks

computersavvy · 11-12-2020, 07:54 PM

Quote:

Originally Posted by GPGAgent

Cheers, I should have been more explicit, I meant one command with no intermediate variable

If you are looking at passing the filename to your command instead of hard coding it (as in a script) then you will need to use the default bash command line internal variables $0, $1, etc. anyway. One of the major strengths of bash is that it allows the use of variables either on the command line or in scripts.

With that said,

Code:

 ls *.mp4 | sed -n s/.mp4$//p | grep -o ....$

will give you the last 4 characters of every filename ending with .mp4 after stripping the "extension" off it. Without using a variable as asked.

GPGAgent · 11-13-2020, 07:58 AM

Quote:

Originally Posted by computersavvy

If you are looking at passing the filename to your command instead of hard coding it (as in a script) then you will need to use the default bash command line internal variables $0, $1, etc. anyway. One of the major strengths of bash is that it allows the use of variables either on the command line or in scripts.

With that said,

Code:

 ls *.mp4 | sed -n s/.mp4$//p | grep -o ....$

will give you the last 4 characters of every filename ending with .mp4 after stripping the "extension" off it. Without using a variable as asked.

Another fine solution, cheers

I like examples because I go over them to see how they work and this helps me write and configure my own versions to suit.

pan64 · 11-13-2020, 08:17 AM

backtick, basename, sed, grep are all new processes, they cost much more (in resources, like time, cpu, ram) than the built-in bash alternatives.
So why do you want a oneliner?

Code:

echo ${ff: -8: -4}

computersavvy · 11-13-2020, 10:02 AM

Quote:

Originally Posted by pan64

backtick, basename, sed, grep are all new processes, they cost much more (in resources, like time, cpu, ram) than the built-in bash alternatives.
So why do you want a oneliner?

Code:

echo ${ff: -8: -4}

I don't know why, but GPGAgent specifically asked for no variable in the one-liner. The only way I know to do that requires chaining processes together instead of using the built-in variable tools of bash. Yours works cleanly and efficiently but does require a variable be used.

GPGAgent · 11-14-2020, 08:15 AM

Quote:

Originally Posted by computersavvy

I don't know why, but GPGAgent specifically asked for no variable in the one-liner. The only way I know to do that requires chaining processes together instead of using the built-in variable tools of bash. Yours works cleanly and efficiently but does require a variable be used.

When I said a one liner, I really meant no intermediate variables or even piping temp results to another process, and this solution is what i was looking for but couldn't get my head around the double negative as it were:

Code:

$ ff=12345678.mp3
jonke@charlie:~$ echo ${ff: -8: -4}
5678
jonke@charlie:~$

This perfect and yes I did know that the syntax for strings is

Code:

${ff:start:length}

so using a negative start point and a negative length is obviously going to work, I just wasn't thinking in a negative way!!!

So -8 means start 8ch in from the end so

Code:

jonke@charlie:~$ echo ${ff: -8:}

jonke@charlie:~$

aha, returns nothing because you're not specifying a length,so lets go for 8ch length

Code:

jonke@charlie:~$ echo $ff   ${ff: -8:8}
1234567890987654321abcd.mp3 abcd.mp3
jonke@charlie:~$

but I only want 4ch

Code:

jonke@charlie:~$ echo $ff   ${ff: -8:4}
1234567890987654321abcd.mp3 abcd
jonke@charlie:~$

so there we have it

Code:

echo $ff   ${ff: -8:4}

You don't need the length to be negative - obvious when you break it down, cheers to everyone who replied.

pan64 · 11-14-2020, 09:52 AM

Quote:

Originally Posted by GPGAgent

So -8 means start 8ch in from the end so

Code:

jonke@charlie:~$ echo ${ff: -8:}

jonke@charlie:~$

aha, returns nothing because you're not specifying a length,so lets go for 8ch length

Code:

jonke@charlie:~$ echo $ff   ${ff: -8:8}
1234567890987654321abcd.mp3 abcd.mp3
jonke@charlie:~$

you do not need the : at the end:

Code:

echo ${ff: -8}

GPGAgent · 11-14-2020, 09:57 AM

Quote:

Originally Posted by pan64

you do not need the : at the end:

Code:

echo ${ff: -8}

That's correct, left it in by mistake - cut 'n paste and didn't check it, btw I should mention the space is important and this is the alternate a\nd perhaps more accurate syntax

Code:

jonke@charlie:~$ echo ${ff:(-8)}  # alternate syntax
5678.mp3
jonke@charlie:~$ echo ${ff: -8}   # space before -8 is important
5678.mp3
jonke@charlie:~$ echo ${ff:-8}    # no, incorrect
12345678.mp3
jonke@charlie:~$

cheers

pan64 · 11-14-2020, 10:08 AM

echo ${ff:-8} is correct, just it has a different meaning, see: ${parameter:-word}