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I have strings that initially contain different directory paths, where both the 2nd and 2nd last sub-directories can vary in length, like so

 /home/Leo/Work/CMI/ARCH/MWS/Disks
 /home/Cleo/Work/CMI/ARCH/BK/Disks

I want to trim the first 5 sub-directories and only show the last 2, like so

 echo "/MWS/Disks"
 echo "/BK/Disks"

One way to trim the first 5 sub-directories from the initial strings might be to left-shift each character until both strings start with the second last '/'.

The Bash Beginners Guide describes a shift built-in that left-shifts positional parameters in a command and throws away unused arguments. But it is not immediately obvious whether this could be used to trim the first 5 sub-directories from the strings described above.

In Bash, how do I reduce these strings, preferably without using loops ?

CLARIFICATION

Judging from comments a bit more context is needed. My Bash script recovers historic Mdos and Qdos files from 8-inch floppy disk images and saves files to directories on the hard drive.

For better or worse, I created a bespoke scheme that stores directory paths using 3-character variable names where each name is an acronym for the section of the path to the current directory.

For example MWC is an acronymn for $MY/Work/CMI in the following path

MY="$USER"
MWC="C:/cygwin64/home/$MY/Work/CMI"
cd "$MWC"
pwd
C:/cygwin64/home/$MY/Work/CMI

Similarly 3-character variables point to the next sub-directory further up the tree

WCA="$MWC/ARCH"

i.e. C:/cygwin64/home/$MY/Work/CMI/ARCH, path to a gallery of archive owners.

As directory paths lengthen the 3-character variables make paths easily identified by conserving white space in the listing. Nevertheless the full path appears whenever my script references a path. Hence the need to trim parts of the string that have no interest for the end user.

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4
  • Do you have a possibility to use sed? echo /home/Leo/Work/CMI/ARCH/MWS/Disks | sed 's#^\(/[^/]*\)\{5\}##' Commented Nov 6, 2021 at 22:12
  • yes though this example didn't work Commented Nov 6, 2021 at 22:42
  • I tried it online Commented Nov 6, 2021 at 22:57
  • how are you processing this list of directories/files? are they coming from a file? a stream from a separate OS process? one-by-one via a variable (eg, in a loop)? Commented Nov 6, 2021 at 23:11

4 Answers 4

Reset to default
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If the number of subdirectories is always the same, you can use parameter expansion to remove the first 5 subdirectories:

s=/home/Leo/Work/CMI/ARCH/MWS/Disks
s=/${s#/*/*/*/*/*/}
echo $s  # /MWS/Disks

Or, if you know you need the last two parts whatever the depth of the path is:

s=/home/Leo/Work/CMI/ARCH/MWS/Disks
last=/${s##*/}
last_but1=${s%$last}
last_but1=/${last_but1##*/}
echo $last_but1$last  # /MWS/Disks
  • ${s#PATTERN} removes PATTERN from the beginning of $s.
  • ${s%PATTERN} removes PATTERN form the end of $s.
  • with # or %, the shortest match of PATTERN is found. Doubling them makes the match the longest possible.
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7 Comments

the last two parts are needed so the user (named in the 2nd sub-directory) knows whose disk archive they are working on (i.e. the archive owner named in the 2nd last sub-directory) - if that makes sense.
@Greg: I rephrased the comments to explain what the difference between the two approaches is. Both of them would probably work for you.
This is good. I'm checking each. Do you have a reference on parameter expansion ?
man bash describes it a bit tersely, so experiment with it to understand the nuances :-)
You're dead right. It took a bit of experimentation to see how this answer addressed the question. There's no way I would have got there by reading man bash. The three bullet-points really helped.
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3

As an alternative to the parameter expansion, you can use the =~ operator:

dir='/home/Leo/Work/CMI/ARCH/MWS/Disks'
[[ $dir =~ /[^/]*/[^/]*$ ]] && echo "${BASH_REMATCH[0]}"
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3 Comments

I not sure what you changed but this worked both before and after the edit. Can you recommend a reference on the =~ operator ?
@Greg Bash Reference Manual, Conditional constructs. Search for the =~ in the page.
@Greg Also, Regular Expressions
1

Assuming the inputs are coming from a file (or streamed/piped from another OS process) ...

Sample input:

$ cat dir.file
 /home/Leo/Work/CMI/ARCH/MWS/Disks
 /home/Cleo/Work/CMI/ARCH/BK/Disks

One awk idea:

awk 'BEGIN {FS=OFS="/"} {print OFS $(NF-1),$(NF)}' dir.file

This generates:

/MWS/Disks
/BK/Disks

If the results need to be stored for later use it shouldn't be too hard to add some code as needed (eg, redirect to a file, pipe to another process, pass as input to a while/read loop, load into an array, etc).

If OP is processing these strings one at a time (eg, as a variable in a loop), I'd probably stick with a parameter substitution solution (see choroba's answer) which doesn't require any overhead to spawn subprocesses.

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1 Comment

See my edits. They answer your questions. Your awk solution is working but I'll confirm when I have tried this in my script
1

Cygwin has find right?

So can you do:

cd 'C:/cygwin64/' # I guess?

find "/home/$USER/work/CMI/ARCH" -mindepth 2 -maxdepth 2 -type d -name Disks |
sed 's=/Disks$=='

Or this, to list all of them:

find /home/*/work/CMI/ARCH/*/ -type d -name Disks |
sed 's=/Disks$=='

If you just want to edit the string, you can use prefix removal:

$ path=/home/Leo/Work/CMI/ARCH/MWS/Disks
$ path=/${path#/*/*/*/*/*/*}
$ echo "$path"
/MWS/Disks
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