Escape characters from a bash variable

BurninLeo

11/5/23, 8:01 PM

I am trying to search-and-replace strings in a configuration template, using bash variables.

Bash variables

BACKUP_KEY="ABCDEFG"
BACKUP_ROOT="/var/backup"

Configuration template (template.conf):

GPG_KEY='BACKUP_KEY_ID'
GPG_OPTS="--pinentry-mode loopback"
TARGET='file://BACKUP_ROOT/partA'
SOURCE='/partA'
MAX_AGE=3M

Expected outcome:

GPG_KEY='ABCDEFG'
GPG_OPTS="--pinentry-mode loopback"
TARGET='file:///var/backup/partA'
SOURCE='/partA'
MAX_AGE=3M

This is easy for the first parameter, using sed, but the backslash in the second parameter needed to be escaped for sed.

# What I try
sed "s/BACKUP_KEY/${BACKUP_KEY}/g; s/BACKUP_ROOT/${BACKUP_ROOT}/g" template.conf > ~/.duply/partA/conf
# What this becomes
sed "s/BACKUP_KEY/ABCDEFG/g; s/BACKUP_ROOT//var/backup/g" template.conf > ~/.duply/partA/conf

I found the bash extension that would allow adding single-quotes, i.e. ${BACKUP_ROOT@Q}, but I did not find an option so far to escape the backslash in the BACKUP_ROOT parameter.

I wonder if there is any elegant option to achieve that? Note, that I need the un-escaped variables in several other places, so I would prefer not to additionally define them with escape backslashes BACKUP_ROOT_ESC="\\/var\\/backup". That would definitly not be elegant ;)

I would also prefer not to use a different limiter, because I also have other variables, for example, passwords that may contain other characters accidently matching the search/replace delimiters. So, finding a way to auto-escape the slash (and maybe also the backslash) would be the most reliable option.

1103

1 + 0

bash

scripts

sed

Score:4

Ubuntu

steeldriver

11/5/23, 8:46 PM

As a matter of style, you should avoid all caps variable names

It's not an expansion flag like Q, but you could use the generic ${parameter//pattern/string} with pattern set to a literal slash \/ and string set to a literal backslash - literal slash \\\/:

$ echo "${BACKUP_ROOT}"
/var/backup

$ echo "${BACKUP_ROOT//\//\\\/}"
\/var\/backup

$ echo "TARGET='file://BACKUP_ROOT/partA'" | sed "s/BACKUP_ROOT/${BACKUP_ROOT//\//\\\/}/"
TARGET='file:///var/backup/partA'

+ 1

BurninLeo

11/6/23, 8:16 PM

Crazy what one can do in the shell :D Thank you very much!

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Escape characters from a bash variable

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