Catch specific string using regex - regex

Question

I have multiple boards. Inside my bash script, I want to catch my root filesystem name using regex. When I do a cat /proc/cmdline, I have this:
BOOT_IMAGE=/vmlinuz-5.15.0-57-generic root=/dev/mapper/vgubuntu-root ro quiet splash vt.handoff=7
I just want to select /dev/mapper/vgubuntu-root
So far I have managed to catch root=/dev/mapper/vgubuntu-root using this command
\broot=[^ ]+

Answer 1

You can use your regex in sed with a capture group:
sed -E 's~.* root=([^ ]+).*~\1~' /proc/cmdline
/dev/mapper/vgubuntu-root
Another option is to use awk(should work in any awk):
awk 'match($0, /root=[^ ]+/) {
print substr($0, RSTART+5, RLENGTH-5)
}' /proc/cmdline
# if your string is always 2nd field then a simpler one
awk '{sub(/^[^=]+=/, "", $2); print $2}' /proc/cmdline

Answer 2

1st solution: With your shown samples in GNU awk please try following awk code.
awk -v RS='[[:space:]]+root=[^[:space:]]+' '
RT && split(RT,arr,"="){
print arr[2]
}
' Input_file
2nd solution: With GNU grep you could try following solution, using -oP options to enable PCRE regex in grep and in main section of grep using regex ^.*?[[:space:]]root=\K\S+ where \K is used for forgetting matched values till root= and get rest of the values as required.
grep -oP '^.*?[[:space:]]root=\K\S+' Input_file
3rd solution: In case your Input_file is always same as shown samples then try this Simple awk using field separator(s) concept.
awk -F' |root=' '{print $3}' Input_file

Answer 3

If the second field has the value, using awk you can split and check for root
awk '
{
n=split($2,a,"=")
if (n==2 && a[1]=="root"){
print a[2]
}
}
' file
Output
/dev/mapper/vgubuntu-root
Or using GNU-awk with a capture group
awk 'match($0, /(^|\s)root=(\S+)/, a) {print a[2]}' file

Answer 4

Since you are using Linux, you can use a GNU grep:
grep -oP '\broot=\K\S+'
where o allows match output, and P sets the regex engine to PCRE. See the online demo. Details:
\b - word boundary
root= - a fixed string
\K - match reset operator discarding the text matched so far
\S+ - one or more non-whitespace chars.

Answer 5

another awk solution, using good ole' FS / OFS :
-- no PCRE, capture groups, match(), g/sub(), or substr() needed
echo 'BOOT_IMAGE=/vmlinuz-5.15.0-57-generic root=/dev/mapper/vgubuntu-root ro quiet splash vt.handoff=7' |
mawk NF=NF FS='^[^=]+=[^=]+=| [^/]+$' OFS=
/dev/mapper/vgubuntu-root
if you're very very certain the structure has root=, then :
gawk NF=NF FS='^.+root=| .+$' OFS=
/dev/mapper/vgubuntu-root
if you like doing it the RS way instead :
nawk '$!NF = $NF' FS== RS=' [^/]+\n'
/dev/mapper/vgubuntu-root