I have 10 text files and I want to paste
each file with its pair, such that I have 5 total files.
I tried the following:
for i in 4_1 5_1 6_1 7_1 8_1
do
for j in 4_2 5_2 6_2 7_2 8_2
do
paste ${i}.txt ${j}.txt > ${i}.${j}.txt
done
done
However, this code combines every possible combination instead of just combining the matching pairs.
So I would like file 4_1.txt
to be paired with 4_2.txt
, 5_1.txt
with 5_2.txt
, etc.
You can use an associative array:
This allows you traversing pairs, triples, etc. Credits: the original idea is taken from @CharlesDuffy-s answer.
If you want to use one variable and perform and action with it, you just need to use one loop:
This will do
There is a common pattern where you have pairs of files, where one name of the pair can be easily derived from the other. If the file you know the name of is X and the other file is Y, you have the following common use cases.
All of these lend themselves to the same rough code skeleton.
The key here is the observation that
y
can be derived fromx
with some simple variable substitutions. So you loop over thex
values, and figure out the correspondingy
value inside the loop.Here, we have used the shell's built-in
${variable#prefix}
and${variable%suffix}
operators to return the variable's value with any leadingprefix
or trailingsuffix
, respectively, trimmed off. (There is also##
and%%
to match the longest, instead of the shortest, possible match. The expression after#
or%
is a regular shell glob pattern.) These should usually be all you need, although you frequently seesed
orawk
scripts even for this trivial job (where really you should usually try to avoid an external process), as well as of course for more demanding transformations.If you need to loop over
x
files scattered across different directories, maybe the loop should start with something likeA commonly seen problem in similar questions is answers which fail to quote
$x
and$y
correctly. Generally, any variable containing a file name should always be in double quotes.Where X and Y are unrelated, a common solution is to loop over a here document containing the mapping:
I agree with the answer currently proposed by fedorqui in the context of the question currently asked. The below is given only to provide some more general answers.
One more general approach (for bash 4.0 or newer) is to store your pairs in an associative array:
Another (compatible with older releases of bash) is to use more than one conventional array:
the above did not work for me, but the following does read values in pairs from an ordered list
(can be more than pairs adding extra 'read-lines' :-)
produces