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If - how to use

Posted 12-05-2008 at 02:27 AM by kshfi
Updated 12-07-2008 at 09:57 AM by kshfi

Tags bash, ksh

Programmers - shell if is command.

Code:

if command exit code is 0
then
    do something 1
    do something 2
fi

Ex.

Code:

if  cp fromfile tofile
then
   echo "Works fine"
else
   echo "Error ???"
fi

[ = test
You can write test or [

Code:

a=1
b=1
test "$a" = "$b"
echo $?
0

a=1
b=2
test "$a" = "$b"
echo $?
1

is same as

Code:

a=1
b=1
[ "$a" = "$b" ]
echo $?
0

a=1
b=2
[ "$a" = "$b" ]
echo $?
1

In some *nix systems test and [ are same binary. Bash, ksh, ... [ and test are built-in commands.

If you like to test command exit status, then it is possible with if.

Code:

a=1
b=1
if [ "$a" = "$b" ] ; then
   echo "equal"
fi
# or
a=1
b=1
if test "$a" = "$b"  ; then
   echo "equal"
fi

You can write it also:

Code:

a=1
b=1
[ "$a" = "$b" ] 
if [ $? = 0 ] ; then
   echo "equal"
fi

Why ; before then ?
if and then on same line = you need
command delimeter ;

Remember
if is command which has arguments and options. = space between every arguments.

Basic string testing
= equal
!= not equal

Basic numeric testing (options)
-eq equal
-ne not equal
-lt less than
-le less or equal
-gt greater than
-ge greater or equal

Basic file testing (options)
-f file exist
-d directory exist
-x execute priviledges
-r read ...
-w write ...

Negation !

Code:

if [ ! -f file ] ; then
   echo "file $file not exists"
fi

Code:

if ! cp x y ; then
   echo "error"
fi

And, or
-a and
-o or

Code:

if cp x y -a cp b z ; then
   echo "noth copy done"
else
   echo "co not worked
fi

if not like to see errors

Code:

if cp x y 2>/dev/null -a cp b z 2>/dev/null; then
   echo "both copy done"
else
   echo "cp not worked"
fi

&& ||

Code:

cp x y && cp a b || echo "error"

Is same as

Code:

if cp x y ; then
  if ! cp a b ; then
        echo "error"
  fi
fi

&& if prev. exit stat 0, then this
|| if prev. exit stat <> 0, then this

Using variables
when use variables in test commands, use "". Why ? If variable is empty - you get error.

Code:

if [ "$a" = "" ] ; then
   echo "empty"
   exit 
fi

If $a value is empty and you write

Code:

if [ $a = "" ] ; then
   echo "empty"
   exit 
fi

you get error, because arg-1 is nothing.
"$a" is string, length 0

[[ ]] (( ))
[[ is not same as [
(( is for only numeric testing (C-like)

Code:

a=1
b=2
((  a>b  )) && echo "$a > $b "

Posted in Sh handbook, syntax, FAQ

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