Workarounds for not being able to use variables in a sequence brace expression:
If the intent is merely to iterate over numbers in a range - as in the OP's case - the best choice is not to use brace expansion, but instead use bash's C-style loop - see user000001's answer.
- If the specific numbers aren't important and you simply need to execute a loop body a specified number of times, Cole Tierney's answer is an option.
If use of brace expansion is desired nonetheless:
If you do NOT need the numbers in the list to have a prefix or postfix, use the seq utility with an unquoted command substitution (small caveat: seq is NOT a POSIX utility, but it is widely available); e.g.
echo $(seq 3) -> 1 2 3; start number 1 implied
echo $(seq -f '%02.f' 3) -> 01 02 03 - zero-padded
echo $(seq 2 4) -> 2 3 4; explicit start and end numbersecho $(seq 1 2 5) -> 1 3 5; custom increment (the 2 in the middle)
If you DO need the numbers in the list to have a prefix or postfix, you have several choices:
- Use the
seq utility with its -f option for providing a printf-style format string (as used above for zero-padding), or pure Bash workarounds based on eval (extra care needed!) or building an array in a loop, all of which are detailed in this answer.
- You could also consider implementing the functionality generically, such as by writing a custom shell function or a custom script with utilities such as
awk or perl.
Example of safe use of eval with variables driving a sequence brace expression:
The variables are validated beforehand, to make sure they contain decimal integers.
from=1 to=3 # sample values
# Ensure that $from and $to are decimal numbers and abort, if they are not.
(( 10#$from + 10#$to || 1 )) 2>/dev/null || { echo "Need decimal integers" >&2; exit 1; }
eval echo "A{$from..$to}" # -> 'A1 A2 A3'
General overview of brace expansion
The main purpose of brace expansion is to expand to a list of tokens with each token having an optional prefix and/or postfix; brace expansions must be unquoted and come in 2 flavors:
- a fixed series (list) of comma-separated strings - variables supported
- specifies and expands to a fixed number of tokens (2 or more); e.g.:
echo A{b,c,d} -> Ab Ac Ad, i.e., 3 tokens, as implied by the number of args.echo {/,$HOME/}Library e.g., -> /Library /User/jdoe/Library- Variable references - and even globs - are supported, but note that they get expanded after brace expansion, in its result, in the course of normal evaluation.
a sequence expression (range) with .., typically numerical - variables NOT supported
- expands to a variable number of tokens, driven by literal start and end points (for historical reasons, use of variables is NOT supported - see the comments on user000001's answer):
- [rare] strings: only single English letters allowed; e.g.
{a..c}
- numbers: decimal integers only; e.g.,
{1..10}, {10..1}, {-1..2}
- example with prefix and postfix:
A{1..3}# -> A1# A2# A3#
- broken example with variables:
{$from..$to} # !! FAILS - $from and $to are interpreted as literals and therefore not recognized as either a single letter or a decimal integer - no brace expansion is performed (see below).
- by contrast, using variables does work in
zsh and ksh.
- bash 4+ adds two features:
- optional increment step value:
echo A{1..5..2} -> A1 A3 A5 - numbers incremented by 2
- ability to zero-pad:
echo A{001..003} -> A001 A002 A003
An invalid brace expression is not expanded (treated like a regular unquoted string, with { and } treated as literals):
echo {} -> '{}' - invalid as a brace expr.: at least 2 ,-separated tokens needed
- This allows the use of unquoted
{} with find, for instance.
echo {1..$to} -> '{1..}' - invalid as a brace expr. in bash: variables not supported; however, valid in ksh and zsh.- (
fish, by contrast, expands any {...} sequence; similarly, zsh has option BRACE_CCL (OFF by default) for expanding individual characters inside {..}, which effectively causes expansion of any nonempty {...} sequence.)
