I have a a.md file:

Start file

```
replace content here by a variable or result of a command such as pwd,
the whole content in this code block should be something like /Users/danny/MyProjects/CurrentDir
```
End file

And a script in update.sh to update file above:

#!/bin/sh

t=$(pwd)
echo $t
perl -i -p0e 's/(?<=```shell\n)[\s\S]*(?=\n```)/$t/s' a.md

It works fine with a string but I cannot replace with a variable such as $(pwd).

CodePudding user response：

A Perl command-line program in that shell script cannot use variables from the script just so but we need to pass variables to it somehow.

There are a few ways to do that,^† and perhaps using -s switch is simplest here

#!/bin/bash

t=$(pwd)
echo $t
perl -i -0777 -s -pe's/(?<=```shell\n)[\s\S]*(?=\n```)/$d/s' -- -d="$t" a.md

# or
# perl -i -0777 -s -pe'...' -- -d="$(pwd)" a.md

The -s for perl enables a basic support for switches for the program itself.

So that -d=... becomes available as $d in the program, either with the assigned value, here the value of the bash variable $t, or 1 if not assigned (-d). We can pass multiple variables this way, each in its own switch.

The -- after the program mark the start of arguments.

Note that we do not need a shell variable but can directly substitute a command output, -var="$(pwd)", if desired, and then $var is available in the Perl program.

^† Aside from using storage (files, databases, etc) or pipes, there are two more ways to directly pass arguments given to this command-line ("one-liner") program

• Pass the variable as an argument and read it from @ARGV in the program

  t=$(pwd)
  perl -i -0777 -s -pe'BEGIN { $d = shift }; s/.../$d/s' "$t" a.md
  

  We need to also remove it from @ARGV so that the files can then be processed, which is what shift does, and need to do this in a BEGIN block since -p sets a loop.

• Export a variable in bash making it an environment variable and a Perl script can then use that via %ENV variable

  export t=$(pwd)
  perl -i -0777 -s -pe's/.../$ENV{t}/s' a.md
  

  Note, it's t in the %ENV hash ($ENV{t}), the name of the variable, not $t (value)

See for example this post and this post