I have a simple BASH command that looks like
for i in `seq 2`; do echo $i; done; > out.dat
When this runs the output of seq 2 is output to the terminal and nothing is output to the data file (out.dat)
I am expecting standard out to be redirected to out.dat like it does simply running the command seq 2 > out.dat
Remove your semicolon.
for i in `seq 2`; do echo "$i"; done > out.dat
SUGGESTIONS
Also as suggested by Fredrik Pihl, try not to use external binaries when they are not needed, or at least when practically not:
for i in {1..2}; do echo "$i"; done > out.dat
for (( i = 1; i <= 2; ++i )); do echo "$i"; done > out.dat
for i in 1 2; do echo "$i"; done > out.dat
Also, be careful of outputs in words that may cause pathname expansion.
for a in $(echo '*'); do echo "$a"; done
Would show your files instead of just a literal *.
$() is also recommended as a clearer syntax for command substitution in Bash and POSIX shells than backticks (`), and it supports nesting.
The cleaner solutions as well for reading output to variables are
while read var; do
...
done < <(do something)
And
read ... < <(do something) ## Could be done on a loop or with readarray.
for a in "${array[@]}"; do
:
done
Using printf can also be an easier alternative with respect to the intended function:
printf '%s\n' {1..2} > out.dat
seq instead of for i in {1..2}; do ...?
(( i = 1 ; i <= 2; ++i )), or simply in 1 2 for compatibility. My answer is only about the problem with redirection, although I tried to fix one: "$i".
done statement to other things like sort, for example for i in foo bar baz; do echo $i | base64; done | sort | base64 -dAnother possibility, for the sake of completeness: You can move the output inside the loop, using >> to append to the file, if it exists.
for i in `seq 2`; do echo $i >> out.dat; done;
Which one is better certainly depends on the use case. Writing the file in one go is certainly better than appending to it a thousand times. Also, if the loop contains multiple echo statements, all of which shall go to the file, doing done > out.dat is probably more readable and easier to maintain. The advantage of this solution, of course, is that it gives more flexibility.
Try:
(for i in `seq 2`; do echo $i; done;) > out.dat
{} when it's not necessary to create another process for the subshell and keeping the variable changes from the higher block. A situation only when it can't be helped is when you place them on a pipeline since they are placed on a subshell by default, where not laspipe is set and commands are on the end, although using process substitutions are the way to go for it.
for loop is already a compound command that groups its body; the subshell here is pointless.