Catch specific string using regex-CodePudding

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 managed to catch root=/dev/mapper/vgubuntu-root using this command

\broot=[^ ]

CodePudding user response：

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=[^ ] /) {
   s = substr($0, RSTART, RLENGTH)
   sub(/^[^=] =/, "", s)
   print s
}' /proc/cmdline

# if your string is always 2nd field then a simpler one
awk '{sub(/^[^=] =/, "", $2); print $2}' /proc/cmdline

CodePudding user response：

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

CodePudding user response：

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.

CodePudding user response：

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