Shell command to loop over names of output file

Question

I have a C code and I use it to extract file and write in to separate files such as exax0.txt, ..., exax202.txt. To do that I use

for i in $(seq 0 202); 
do echo $i | ./f.out exa-NO-hyp.bin exax${i}.txt <<< "${i}"
done

Instead, I would like to have my output files named as exax495.txt, exax535.txt, ..., exax8545.txt. To do so, I tried something like:

for i in $(seq 0 202); 
do echo $i | ./f.out exa-NO-hyp.bin exax${(i*40+495)}.txt <<< "${i}"  
done

But it says, -bash: exax${i*40}.txt: bad substitution

Can anybody please help me with this?

Note that echo "$i" | program ... <<< "$i" is wasting the pipe. The input ends up coming from the here string. Drop the echo "$i" | part. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 27, 2015 at 17:05
No harm to spend an extra line of code: something like filenumber=xxx and ./f.out exa-no-hyp.bin exa${filenumber}.txt. — Walter A
– Walter A, Commented Apr 27, 2015 at 17:36

Sebastian Paaske Tørholm · Accepted Answer · 2015-04-27 17:45:56Z

2

You can use $((...)) to perform arithmetic.

I.e. a simple use would be:

for i in {1..10}; do touch file$(($i+5)).txt ; done

Or in your case:

for i in $(seq 0 202); do echo $i | ./f.out exa-NO-hyp.bin exax$(($i*40+495)).txt <<< "${i}"; done

1 Comment

Etan Reisner Over a year ago

$ is not necessary on variables in $((...)) blocks. and a reference to the Arithemetic Expansion section of the bash reference manual is likely a better than a reference to the (often poor) TLDP ABS guide.

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