Swap characters in a string using shell script

Question

I need to swap characters of a string (which is mmddyyyy format) and rearrange them in yyyymmdd. This string is obtained from a file name (abc_def_08032011.txt).

string=$(ls abc_def_08032011.txt | awk '{print substr($0,9,8)}')

For example:

Current string: 08032011 (This may not necessarily be the current date)
Desired string: 20110803

I tried split function, but it won't work since the string does not have any delimiter.

Any ideas/suggestions greatly appreciated.

bash, tcsh, or any other shell?

Shi
– Shi

2011-08-03 19:47:11 +00:00
Commented Aug 3, 2011 at 19:47 — Shi
– Shi, Commented Aug 3, 2011 at 19:47

Keith Thompson · Accepted Answer · 2011-08-03 19:50:40Z

4

echo 08032011 | sed 's/$....$$....$/\2\1/'

or

echo 08032011 | perl -pe 's/(....)(....)/$2$1/'

answered Aug 3, 2011 at 19:50

Keith Thompson

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2 Comments

Keith Thompson Over a year ago

@pinhead: It took me just a moment to realize that that's your user name. 8-)} Note that it's quite inflexible. There are a number of other possibilities that, for example, search for sequences of digits. But if you're sure what the input looks like and don't need error checking, that's fine.

pinhead Over a year ago

the input will pretty much stay in the same format, so we need not worry about error checking. I am still new to using sed, it would really help if you could explain me how the extended regular expressions work in your given solution?Thanks.

jfg956 · Accepted Answer · 2011-08-04 10:11:48Z

1

Why not using awk all the way:

echo abc_def_08032011.txt | awk '{print substr($0,13,4) substr($0,9,4)}'

or sed all the way, avoiding one awk:

echo abc_def_08032011.txt | sed 's/^........\(....\)\(....\).*$/\2\1/'

or using ksh substitution all the way to avoid spawning a awk/sed process:

s=abc_def_08032011.txt
s1="${s#????????}"
s2="${s1%.*}"
echo "${s2#????}${s2%????}"

answered Aug 4, 2011 at 10:11

jfg956

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