One way:

sed ':a;s/^.\{1,29\}$/&0/;ta;n' file

Match any character(.), and match 1 to 29 characters of the same type(1,29). If match successful, put a '0' behind the pattern matched(&). When the match fails which means when the count of characters excluding the first character exceeds 29, stop it and hence we get the string zero padded by 30.

n at the end simply reads the next line and prints it. This way the even numbered lines get printed untouched.

Executing the above script:

$ sed ':a;s/^.\{1,29\}$/&0/;ta;n' file
FSDFDSFSD000000000000000000000
FDSFD
FHGDHFDHGHFGHGHGF0000000000000
HHGDF
GFDGDFGFDG00000000000000000000
GFDGFDGFDGDFGDGD

One way:

sed -e :a -e 's/^.\{1,29\}$/&0/;ta' -e n file

Match any character(.), and match 1 to 29 characters of the same type(1,29). If match successful, put a '0' behind the pattern matched(&). When the match fails which means when the count of characters excluding the first character exceeds 29, stop it and hence we get the string zero padded by 30.

n at the end simply reads the next line and prints it. This way the even numbered lines get printed untouched.

Executing the above script:

$ sed -e :a -e 's/^.\{1,29\}$/&0/;ta' -e n file
FSDFDSFSD000000000000000000000
FDSFD
FHGDHFDHGHFGHGHGF0000000000000
HHGDF
GFDGDFGFDG00000000000000000000
GFDGFDGFDGDFGDGD

One way:

sed ':a;s/^.\{1,29\}$/&0/;ta;n' file

One way:

sed ':a;s/^.\{1,29\}$/&0/;ta;n' file

Match any character(.), and match 1 to 29 characters of the same type(1,29). If match successful, put a '0' behind the pattern matched(&). When the match fails which means when the count of characters excluding the first character exceeds 29, stop it and hence we get the string zero padded by 30.

n at the end simply reads the next line and prints it. This way the even numbered lines get printed untouched.

Executing the above script:

$ sed ':a;s/^.\{1,29\}$/&0/;ta;n' file
FSDFDSFSD000000000000000000000
FDSFD
FHGDHFDHGHFGHGHGF0000000000000
HHGDF
GFDGDFGFDG00000000000000000000
GFDGFDGFDGDFGDGD