How to trim string and remove characters from string in bash?

Question

I have this:

$ git show -s --format=%ci HEAD
2017-08-21 20:35:28 +0200

How can trim the string at the first space and then remove the dashes, so that I end up with an environment variable with value 20170821?

I tried this, but it didn't work...:

MYDATE=`git show -s --format=%ci HEAD`
MYDATE=${MYDATE%" "} # trim at the first space
MYDATE=${MYDATE//-}. # remove dashes

That's a regular shell variable, not an environment variable. A shell variable only becomes an environment variable if you export it (which you shouldn't do unnecessarily), or if there already existed an environment variable by the same name in your program's environment. — Charles Duffy
– Charles Duffy, Commented Aug 22, 2017 at 16:40
BTW, POSIX-specified convention indicates that all-caps variable names are used for variables with meaning to the shell and operating system; see pubs.opengroup.org/onlinepubs/9699919799/basedefs/…, fourth paragraph, which guarantees that (only) lower-case variable names are safe for use without modifying shell behavior. (This applies to regular shell variables because of their behavior of overwriting like-named environment variables if such exist). — Charles Duffy
– Charles Duffy, Commented Aug 22, 2017 at 16:42
(well -- variable names containing lower-case characters, rather; MyVar is safe, as is myvar, whereas MYVAR is not). — Charles Duffy
– Charles Duffy, Commented Aug 22, 2017 at 19:20

Toby Speight · Accepted Answer · 2017-08-23 07:36:55Z

4

The replacement ${MYDATE%" "} will remove a single space from the end of the string. To match space followed my more text, we need to add a *, and to remove the longest matching substring, we need %% rather than %. We don't need to quote the space in the substitution, so the simplest equivalent is ${MYDATE%% *}.

Demo:

$ MYDATE='2017-08-21 20:35:28 +0200'
$ echo ${MYDATE%% *}
2017-08-21

From a cursory reading of the git show documentation, it appears that you can get the output in the format you want, without having to do any extra work subsequently:

git show -s --format=%cd --date=short HEAD

# Or, using a standard `strftime` format string:
git show -s --format=%cd --date=format:%F HEAD

These both give an ISO-8601 format date; if you want it without the - separators, then you'll need a format string of %Y%m%d:

git show -s --format=%cd --date=format:%Y%m%d HEAD

edited Aug 23, 2017 at 7:36

answered Aug 22, 2017 at 16:24

Toby Speight

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2 Comments

fredrik Over a year ago

Thanks, this is great. I ended up using this: git show -s --format=%cd --date=format:'%Y%m%d' which gives me 20170821.

Toby Speight Over a year ago

Thanks, @BenjaminW. I knew what I meant, but I didn't find the right words!

Ulises André Fierro · Accepted Answer · 2017-08-22 16:33:29Z

2

To manipulate the string so it matches your desired output, use:

${MYDATE/ */}

It'll extract the substring before first space it finds.

Edit: Just to elaborate a little bit more, this is substring replacement, we're just using a short expression that matches everything after the first space and replacing it with nothing. Also, I just noticed the 'remove dashes' portion of the question, you can use the following statement:

tr -d '-' <<< ${MYDATE/ */}

Will result in20170821

edited Aug 22, 2017 at 16:33

answered Aug 22, 2017 at 16:24

Ulises André Fierro

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Comments

nagendra547 · Accepted Answer · 2017-08-22 16:38:20Z

0

Simple way to do- If you don't want to focus much on git utility and consider using bash commands to manipulate the string output - use cut

MYDATE=`git show -s --format=%ci HEAD | cut -d' ' -f1`

answered Aug 22, 2017 at 16:38

nagendra547

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3 Comments

Charles Duffy Over a year ago

None of the other answers are using git-specific commands -- parameter expansion (as in ${varname##" "*}) is bash-native syntax, and much more efficient than forking off a pipeline to run external commands (like cut) that aren't actually part of the shell. See wiki.bash-hackers.org/syntax/pe, and BashFAQ #100. (And on the subject of using modern syntax -- $() is fully POSIX-compliant, and strongly preferred over backticks -- being easier to nest, and avoiding syntax with side effects on how enclosed content is parsed).

nagendra547 Over a year ago

Thanks Charles for info. Good information on parameter expansion. By the way cut is part of GNU coreutils and almost all linux based OS has this utility. I find it quiet handy to manually operate on strings, as it's easy to do. But I agree here on using bash-native syntax that's always faster to do than fork a new pipe.

Charles Duffy Over a year ago

coreutils is, as you imply, specific to Linux -- other operating systems aren't guaranteed to have coreutils-provided tools. That said, cut specifically is also POSIX-specified, which provides a stronger guarantee of cross-platform availability. That said, my point was specifically that cut wasn't a bash built-in, such that bash needs to perform an execv-family syscall to run it, linking and loading a separate executable after forking a process to run it in: the distinction was about performance, not availability.

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