bash replace one character from the end of string

Question

I need to replace last dot to character '-' in the string.

# a='2.5.2.pl'

Using the following expression:

# echo ${a/%./-}

I expect to get:

2.5.2-pl

but i get

2.5.2.pl

I noticed that it doesn't work only if I need to replace the dot from the end to the beginning. Why does it happen? Of course I can use external programs like awk, sed to solve this problem but I need to solve the problem using only bash.

Thanks for advice!

Cyrus · Accepted Answer · 2017-03-12 18:02:26Z

6

With bash‘s Parameter Expansion:

a='2.5.2.pl'
echo "${a%.*}-${a##*.}"

Output:

2.5.2-pl

answered Mar 12, 2017 at 18:02

Cyrus

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Comments

andria_girl · Accepted Answer · 2017-03-12 18:30:54Z

0

My way is a bit hacky and uses rev but I tested it and it works!

echo "$(_b=$(echo "$a" | rev); _b=${_b/./-}; echo "$_b" | rev)"

Basically, I just reversed the character order so the last . was first and then used ${var/./-} to replace the dot with a dash and finally reversed the order of the characters again.

edited Mar 12, 2017 at 18:30

answered Mar 12, 2017 at 18:16

andria_girl

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5 Comments

Charles Duffy Over a year ago

There's quite a set of bugs here. Try having whitespace-surrounded wildcards, tab characters, or other oddness in your strings -- and see BashPitfalls #14. (The ease of having something look like it works, while having bugs when executed with even slightly-different data, is part of why it's important to understand the execution model for bash, rather than accepting a successful test to have meaning besides the specific inputs and domain where that test took place).

andria_girl Over a year ago

I don't see that being a problem with what the OP asked.

Charles Duffy Over a year ago

Code gets reused outside its initial domain. Robust practices are safely reusable; fragile practices... aren't. And reuse isn't always intentional, either -- the worst data loss event I was present for happened when someone was sloppy in quoting in a shell script because it "could only ever" run in a directory containing names matching [0-9a-f]{24} -- then a bug in a separate program created a filename with garbage dumped from memory, and the script expanded a glob in that name and deleted TB of billing data backups.

Charles Duffy Over a year ago

Anyhow, part of the point of StackOverflow is that we're creating a teaching resource -- it's a reasonable expectation that code given in a teaching resource is going to be correct, not full of hidden pitfalls and caveats.

andria_girl Over a year ago

Okay, I'll update it to something that should work outside of it's initial domain.

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