Shell script "for" loop syntax

Question

I have gotten the following to work:

for i in {2..10}
do
    echo "output: $i"
done

It produces a bunch of lines of output: 2, output: 3, so on.

However, trying to run the following:

max=10
for i in {2..$max}
do
    echo "$i"
done

produces the following:

output: {2..10}

How can I get the compiler to realize it should treat $max as the other end of the array, and not part of a string?

Solution 1

Brace expansion, {x..y} is performed before other expansions, so you cannot use that for variable length sequences.

Instead, use the seq 2 $max method as user mob stated.

So, for your example it would be:

max=10
for i in `seq 2 $max`
do
    echo "$i"
done

Solution 2

Try the arithmetic-expression version of for:

max=10
for (( i=2; i <= $max; ++i ))
do
    echo "$i"
done

This is available in most versions of bash, and should be Bourne shell (sh) compatible also.

Solution 3

Step the loop manually:

i=0
max=10
while [ $i -lt $max ]
do
    echo "output: $i"
    true $(( i++ ))
done

If you don’t have to be totally POSIX, you can use the arithmetic for loop:

max=10
for (( i=0; i &lt max; i++ )); do echo "output: $i"; done

Or use jot(1) on BSD systems:

for i in $( jot 0 10 ); do echo "output: $i"; done

Solution 4

If the seq command available on your system:

for i in `seq 2 $max`
do
  echo "output: $i"
done

If not, then use poor man's seq with perl:

seq=`perl -e "\$,=' ';print 2..$max"`
for i in $seq
do
  echo "output: $i"
done

Watch those quote marks.

Solution 5

We can iterate loop like as C programming.

#!/bin/bash
for ((i=1; i<=20; i=i+1))
do 
      echo $i
done

View more solutions

Comments

[whatsisname]

what system and shell are you using? What kind of goofy system has sh or bash, but doesn't have seq, a coreutil?

[Rahul Das]

FreeBSD doesn't.

[a paid nerd]

Small style nit: I usually see the do and then keywords on the same line as for and if, respectively. E.g., for i in {2..10}; do

[Barmar]

possible duplicate of Is it possible to use a variable in for syntax in bash?

[jrm]

FreeBSD, at least 10, does have /usr/bin/seq.

[Charles Duffy]

@whatsisname, seq is not POSIX-standardized. Even if it exists, it's not guaranteed to have any particular behavior. There's a reason none of the patterns discussed in wooledge.org/~greybot/meta/counting (the relevant entry in the freenode #bash channel's factoid database) depend on it.

[system PAUSE]

seq is relatively new. I only found out about it a few months ago. But you can use a 'for' loop!! The disadvantage of a 'while' is that you have to remember to increment the counter somewhere inside the loop, or else loop downwards.

[Flow]

true $(( i++ )) doesn't work in all cases, so most portable would be true $((i=i+1)).

[system PAUSE]

@Flow: Hm, I just tried it on a couple of systems (Linux and BSD based) with #!/bin/sh and it worked fine. Invoked under bash and specifically under /bin/sh, still worked. Maybe the version of sh matters? Are you on some old Unix?

[chepner]

Very few systems have a dedicated sh, instead making it a link to other another shell. Ideally, such a shell invoked as sh would only support those features in the POSIX standard, but by default let some of their extra features through. The C-style for-loop is not a POSIX feature, but may be in sh mode by the actual shell.

[chepner]

A security risk, since eval will evaluate anything you set max to. Consider max="2}; echo ha; #", then replace echo ha with something more destructive.

[logan]

semi colon should not come in "for (( i=0; i < max; i++ ));"

[Conrad Meyer]

(S)he set max to 10. No risk.

[miller]

There is no good reason to use an external command such as seq to count and increment numbers in the for loop, hence it is recommend that you avoid using seq. This command is left for compatibility with old bash. The built-in commands are fast enough. for (( EXP1; EXP2; EXP3 )) ...

[Wernight]

@miller the for (( ... )) syntax isn't POSIX and that means for example that it won't work out of the box on things like Alpine Linux. seq does.

[Franklin Yu]

@Wernight Not just Alpine Linux; #!/bin/sh is Dash in Debian/Ubuntu.