CODEWITHSUNDEEP
linuxbashshellfor-loopdash-shell
mkab
mkab

Reputation: 953

syntax of for loop in linux shell scripting

I have a problem implementing a for loop. I get this error when I execute my script

test1.sh: 2: Syntax error: Bad for loop variable

I don't understand this error.

This is my script

#!/bin/bash
for (( c=1; c<=5; c++ ))
do
echo "Welcome $c times..."
done

can any one tell me syntax for for loop in sh(in ubuntu it links to dash shell) shell in ubuntu?

Upvotes: 21

Views: 35128

Answers (4)

David W.
David W.

Reputation: 107080

Your shell script (as shown) runs in both Korn shell and Bash. Some thoughts:

  • You might need a space after the shebang (#! /bin/bash and not #!/bin/bash). However, Dennis Ritchie had originally specified the space is optional. Besides, it isn't the error you get with Bourne shell (you get syntax error: '(' unexpected instead).
  • Are you on a Windows system? Just a stab in the dark. This doesn't look like a Windows error.
  • Is this Solaris or HP/UX system? They might not be running true versions of Bash, or maybe an older version. However, even the oldest version of Bash recognizes the for ((x;y;z)) construct.

Try this:

#! /bin/bash
set -vx
echo "Random = $RANDOM"   #Test for bash/Kornshell. Will be blank in other shells
echo \$BASH_VERSINFO[0] = ${BASH_VERSINFO[0]} #Should only work in BASH
echo \$BASH_VERSINFO[1] = ${BASH_VERSINFO[1]}
echo \$BASH_VERSINFO[2] = ${BASH_VERSINFO[2]}
echo \$BASH_VERSINFO[3] = ${BASH_VERSINFO[3]}
echo \$BASH_VERSINFO[4] = ${BASH_VERSINFO[4]}
echo \$BASH_VERSINFO[5] = ${BASH_VERSINFO[5]}
for ((c=0, c<=5, c++))
do
    echo "Welcome $c times"
done
  • The set -xv will display all lines as they are executed.
  • The $RANDOM should display a value if this is either BASH or Kornshell (your for loop will work in either one).
  • The {$BASH_VERINFO[x]} should only be set if this is truly BASH. These aren't even set even if you run Korn shell after you're in BASH (unlike $SHELL which will still contain bash).

If the for loop still gives you trouble, just delete it. Somewhere in this script, we'll find out if you're really executing a bash shell or not.

Upvotes: 1

Ken Bloom
Ken Bloom

Reputation: 58800

A standard POSIX shell only accepts the syntax for varname in list

The C-like for-loop syntax for (( expr1; expr2; expr3 )) is a bashism.

You can get similar behavior in the standard POSIX shell using for c in $(seq 1 5)

Upvotes: 6

Ankur Agarwal
Ankur Agarwal

Reputation: 24768

What does 

ls -l /bin/sh

give on your machine ?

Make sh a symbolic link to bash and then you can do sh ./test1.sh

Upvotes: 1

Michael Krelin - hacker
Michael Krelin - hacker

Reputation: 143249

You probably run it with sh, not bash. Try bash test1.sh, or ./test1.sh if it's executable, but not sh test1.sh.

Upvotes: 38

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