{"id":2223,"date":"2022-03-02T06:26:21","date_gmt":"2022-03-02T06:26:21","guid":{"rendered":"https:\/\/blog.samarthya.me\/wps\/?p=2223"},"modified":"2022-03-02T06:27:41","modified_gmt":"2022-03-02T06:27:41","slug":"bash-named-reference","status":"publish","type":"post","link":"https:\/\/blog.samarthya.me\/wps\/2022\/03\/02\/bash-named-reference\/","title":{"rendered":"Bash: Named Reference"},"content":{"rendered":"\n

Bash is different world. Powerful, simple, elegant and complex all at the same time. Recently while trying to reduce the burden of repetitive code, I was trying to check what can be done to use references in Bash (something like pointers) and it was an interesting problem that I enjoyed solving.<\/p>\n\n\n\n

\"\"<\/div>
Bash<\/div><\/div>

The \u2018$<\/samp>\u2019 character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.<\/p><\/div><\/div>\n\n\n\n

declare -n <name><\/code><\/h2>\n\n\n\n

In bash version 5.1.8 (bash --version<\/code>) I could see that you can utilize the local -n<\/code> or declare -n<\/code> to help you define a named reference but in the version before like 4.2.46<\/code> there was no support for named reference<\/p>\n\n\n\n

> bash --version\nGNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)\nCopyright (C) 2011 Free Software Foundation, Inc.\nLicense GPLv3+: GNU GPL version 3 or later <http:\/\/gnu.org\/licenses\/gpl.html>\n\nThis is free software; you are free to change and redistribute it.\nThere is NO WARRANTY, to the extent permitted by law.<\/code><\/pre>\n\n\n\n
declare: declare [-aAfFgilrtux] [-p] [name[=value] ...]\n    Set variable values and attributes.\n    \n    Declare variables and give them attributes.  If no NAMEs are given,\n    display the attributes and values of all variables.\n    \n    Options:\n      -f\trestrict action or display to function names and definitions\n      -F\trestrict display to function names only (plus line number and\n    \tsource file when debugging)\n      -g\tcreate global variables when used in a shell function; otherwise\n    \tignored\n      -p\tdisplay the attributes and value of each NAME\n    \n    Options which set attributes:\n      -a\tto make NAMEs indexed arrays (if supported)\n      -A\tto make NAMEs associative arrays (if supported)\n      -i\tto make NAMEs have the `integer' attribute\n      -l\tto convert NAMEs to lower case on assignment\n      -r\tto make NAMEs readonly\n      -t\tto make NAMEs have the `trace' attribute\n      -u\tto convert NAMEs to upper case on assignment\n      -x\tto make NAMEs export\n    \n    Using `+' instead of `-' turns off the given attribute.\n    \n    Variables with the integer attribute have arithmetic evaluation (see\n    the `let' command) performed when the variable is assigned a value.\n    \n    When used in a function, `declare' makes NAMEs local, as with the `local'\n    command.  The `-g' option suppresses this behavior.\n    \n    Exit Status:\n    Returns success unless an invalid option is supplied or an error occurs.<\/code><\/pre>\n\n\n\n
Example of -n<\/code><\/h2>\n\n\n\n
I defined a function set_var that takes one argument ( a variable name) and sets the value to ‘NewValue<\/code>‘ as shown below.<\/p>\n\n\n\n
\"\"<\/div>
Bash<\/div><\/div>
A variable can be assigned the nameref<\/var> attribute using the -n<\/samp> option to the declare<\/code> or local<\/code> builtin commands (see Bash Builtins<\/a>) to create a nameref<\/var>, or a reference to another variable. <\/p><\/div><\/div>\n\n\n\n
#!\/bin\/bash\n\nfunction set_var {\n    local -n myVar=$1\n    echo \" before: $myVar\"\n    myVar=\"New Value\"\n    echo \" After: ${myVar}\"\n}\n\n\n(( $# < 1 )) && (echo \" Usage: ${0##*\/} <any value>\")  && exit 1;\nOLDVALUE=$1\necho \" Before function invoke: OLDVALUE := ${OLDVALUE}\"\necho \" Invoking the function change\"\nset_var OLDVALUE\necho \" After function invoke: OLDVALUE := ${OLDVALUE}\"<\/code><\/pre>\n\n\n\n
Example of without -n<\/code><\/h2>\n\n\n\n
If I use the code above in the older bash version where -n<\/code> is not supported.<\/p>\n\n\n\n
> .\/script.sh old\n Before function invoke: OLDVALUE := old\n Invoking the function change\n.\/script.sh: line 4: local: -n: invalid option\nlocal: usage: local [option] name[=value] ...\n before: \n After: New Value\n After function invoke: OLDVALUE := old<\/code><\/pre>\n\n\n\n
The basic form of parameter expansion is ${parameter<\/var>}.<\/p>\n\n\n\n
\"\"<\/div>
Bash<\/div><\/div>
If the first character of parameter<\/var> is an exclamation point (!), and parameter<\/var> is not a nameref<\/var>, it introduces a level of indirection. <\/p><\/div><\/div>\n\n\n\n
\"\"<\/div>
Saurabh<\/div><\/div>
Read more about it in the official documentation here<\/a><\/p><\/div><\/div>\n\n\n\n
Let’s improvise and see what we can do<\/p>\n\n\n\n
#!\/bin\/bash\n\nfunction set_var {\n    local myVar=$1\n    echo \" before: ${!myVar}\"\n    eval $myVar='abcdef'\n    echo \" After: ${!myVar}\"\n}\n\n\n(( $# < 1 )) && (echo \" Usage: ${0##*\/} <any value>\")  && exit 1;\necho \" Parameter:  $*\"\nOLDVALUE=$1\necho \" Before function invoke: OLDVALUE := ${OLDVALUE}\"\necho \" Invoking the function change\"\nset_var OLDVALUE\necho \" After function invoke: OLDVALUE := ${OLDVALUE}\"<\/code><\/pre>\n\n\n\n
\"\"<\/div>
Saurabh<\/div><\/div>
Indirection: The expansion ${!prefix<\/var>*}<\/code> expansion, which expands to the names of all shell variables whose names begin with prefix<\/var>, is available.<\/p><\/div><\/div>\n\n\n\n
 > .\/script.sh old\n Parameter:  old\n Before function invoke: OLDVALUE := old\n Invoking the function change\n before: old\n After: abcdef\n After function invoke: OLDVALUE := abcdef<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
Bash is different world. Powerful, simple, elegant and complex all at the same time. Recently while trying to reduce the burden of repetitive code, I was trying to check what can be done to use references in Bash (something like pointers) and it was an interesting problem that I enjoyed solving. Bash The \u2018$\u2019 character […]<\/p>\n","protected":false},"author":1,"featured_media":2226,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"image","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"footnotes":""},"categories":[34,239],"tags":[245,248,112,249],"class_list":["post-2223","post","type-post","status-publish","format-image","has-post-thumbnail","hentry","category-technical","category-technical-2","tag-bash","tag-indirect","tag-linux","tag-nameref","post_format-post-format-image"],"_links":{"self":[{"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/posts\/2223","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/comments?post=2223"}],"version-history":[{"count":0,"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/posts\/2223\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/media\/2226"}],"wp:attachment":[{"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/media?parent=2223"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/categories?post=2223"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.samarthya.me\/wps\/wp-json\/wp\/v2\/tags?post=2223"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}