CODEWITHSUNDEEP
posixsh
benizi
benizi

Reputation: 4216

Nested parameter substitution in POSIX shell

Nesting parameter substitutions works in Zsh:

$ param=abc

# nested remove prefix ${...#a} and remove suffix ${...%c} =>
$ printf '%s\n' ${${param#a}%c}
# => b

Is there any equivalent in POSIX?

$ param=abc
$ printf '%s\n' ${${param#a}%c}
# => dash: 2: Bad substitution
# => sh: ${${param#a}%c}: bad substitution
# => bash: ${${param#a}%c}: bad substitution

Upvotes: 3

Views: 1046

Answers (2)

chepner
chepner

Reputation: 531858

You can use expr instead to extract the text between the desired prefix and suffix. (This is not, of course, a general purpose equivalent to nested expressions, but does solve your given problem.)

param=abc
expr "$param" : "a\(.*\)c"

The regular expression matching operator : of expr takes two arguments: the left argument is a string, the right argument is a regular expression. The output is whatever is matched inside the \(...\) group.

Upvotes: 3

Zombo
Zombo

Reputation: 1

Bash does not, but you have a host of other tools that can get the job done.

cut -b2 <<< abc
tr -d ac <<< abc
sed s/[ac]// <<< abc
awk '$0=$2' FS= <<< abc

It should be noted that parameter substitution does not scale

Parameter expansion slow for large data sets

Upvotes: 1

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