bash脚本替换字符串中的值

Question

I've used awk and grep in the past to extract substrings using a bash script, but what I can't figure out is how to find a substring and then replace part of a value in that substring using a bash script.

If I have a string as follows:

"key val1=0 val2=15 val3=22 'some notes here'"

How can I efficiently update val1, val2, and val3 values to something else? So if I wanted to change "val1=0" to "val1=9999", and "val2=22" to "val2=0".

I could split the string based on spaces, loop through the values to find val1, split it to get 0, change 0 to 9999(repeat for val2), but then how do I recreate the original string with the original values so I wind up with:

"key val1=9999 val2=0 val3=22 'some notes here'"

I have a requirement that this gets done in a bash script, which I am not very familiar with, so switching it over to python, perl, or some other language isn't an option for me unfortunately.

The strings would be passed to the script via STDIN if that makes a difference. The only pieces of the string I am interested in changing is "val#=#", all other text should remain untouched.

Answer 1

Using pure bash

Let's start with your string:

$ s="key val1=0 val2=15 val3=22 'some notes here'"

Now, let's replace, as an example, val1 :

$ s="${s/val1=0/val1=9999}"
$ echo "$s"
key val1=9999 val2=15 val3=22 'some notes here'

The construct ${var/old/new} is called pattern substitution . The value of old can be a character simple string or a shell glob.

Using bash and sed

In bash scripts, sed is often used for string manipulation and it works well with stdin. For example, to perform two substitutions at once:

$ input="key val1=0 val2=15 val3=22 'some notes here'"
$ echo "$input" | sed 's/val1=[^ ]*/val1=9999/; s/val2=[^ ]*/val2=0/'
key val1=9999 val2=0 val3=22 'some notes here'

The output of sed can, of course, be captured in a shell variable using command substitution :

$ new="$(echo "$input" | sed 's/val1=[^ ]*/val1=9999/; s/val2=[^ ]*/val2=0/')"
$ echo "$new"
key val1=9999 val2=0 val3=22 'some notes here'