BASH: Use a String as file output

Question

In a bash script I need to compare the first char of two Strings.

To do this, I use the head operator like this:

var1="foo"
var2="doo"
headVar1=$(head -c1 $var1)
headVar2=$(head -c1 $var2)

if [ $headVar1 == $headVar2 ]
  then
    #miau
fi

But the console says "head: cant open foo for reading: Doesnt exist the file or directorie" And the same with doo

Some help?

Thanks.

You have so many syntax errors. To get the first char use ${var1:0:1} — anubhava
– anubhava, Commented Nov 8, 2015 at 16:29
In addition to all that's said in the answers, == isn't supported in POSIX [. Bash will support it as an extension, but better to use the compliant string comparison operator =. — Charles Duffy
– Charles Duffy, Commented Nov 8, 2015 at 16:42

choroba · Accepted Answer · 2015-11-08 18:52:16Z

3

head interpreted foo as a filename. See man head on what options are available for the command.

To store command output in a variable, use command substitution:

headVar1=$(printf %s "$var1" | head -c1)

which could be shortened using a "here string":

headVar1=$(head -c1 <<< "$var1")

But parameter expansion is much faster in this case, as it doesn't spawn a subshell:

headVar1=${var1:0:1}

edited Nov 8, 2015 at 18:52

answered Nov 8, 2015 at 16:31

choroba

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3 Comments

Charles Duffy Over a year ago

The first form isn't quite right in all cases. printf '%s\n' "$var1" will more reliably emit the content faithfully and correctly than echo "$var1" will.

choroba Over a year ago

@CharlesDuffy: Can you give an example?

Charles Duffy Over a year ago

Sure: var1=-n (in many versions; the POSIX spec makes this behavior implementation-defined). And if you're following the POSIX spec for echo, then any string with a literal backslash likewise results in implementation-defined output.

Avinash Raj · Accepted Answer · 2015-11-08 16:29:53Z

1

Your code must be,

var1="foo"
var2="doo"
headVar1=$(head -c1 <<<"$var1")
headVar2=$(head -c1 <<<"$var2")
if [[ "$headVar1" == "$headVar2" ]]
  then
    echo "It's same"
else
    echo "Not same"
fi

remove the space which exists next to equal sign (variable assignment part).
Use <<< to fed input from a variable to the corresponding function.
Variable names must be preceded by the dollar $ symbol.

answered Nov 8, 2015 at 16:29

Avinash Raj

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Comments

chepner · Accepted Answer · 2015-11-08 19:28:15Z

0

A circuitous, but POSIX-compatible, solution:

var1="foo"
var2="doo"
headVar1=${var1%${var1#?}}
headVar2=${var2%${var2#?}}

if [ "$headVar1" = "$headVar2" ]
then
    #miau
fi

${var1#?} expands to the value of var1 minus its first character. That is used as the suffix to remove with ${var1%...}, leaving in the end just the first character of var1.

answered Nov 8, 2015 at 19:28

chepner

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