How to get first character of variable

Question

I'm trying to get the first character of a variable, but I'm getting a Bad substitution error. Can anyone help me fix it?

code is:

while IFS=$'\n' read line
do
  if [ ! ${line:0:1} == "#"] # Error on this line
  then
    eval echo "$line"
    eval createSymlink $line
  fi
done < /some/file.txt

Am I doing something wrong or is there a better way of doing this?

-- EDIT --

As requested - here's some sample input which is stored in /some/file.txt

$MOZ_HOME/mobile/android/chrome/content/browser.js
$MOZ_HOME/mobile/android/locales/en-US/chrome/browser.properties
$MOZ_HOME/mobile/android/components/ContentPermissionPrompt.js

maybe you need a space between the closing quote and the closing brackets, i.e. `"#" ]` — doubleDown, Jul 18 '13 at 12:45
also you might as well go with `A != B` instead of `! A == B` — doubleDown, Jul 18 '13 at 12:46
@doubleDown neither of those worked - sorry! Still getting a Bad substitution error on that line — Martyn, Jul 18 '13 at 12:52
@Martyn can you post some input data from file.txt so we can test? — fedorqui, Jul 18 '13 at 12:54
Perhaps there is a blank line in the file, or a line starting with whitespace? try with `if [ ! "${line:0:1}" == "#" ]` — user000001, Jul 18 '13 at 12:55
can't reproduce the substitution error, even with the given sample input. — doubleDown, Jul 18 '13 at 13:02
Possible duplicate of [How to obtain the first letter in a Bash variable?](https://stackoverflow.com/questions/10218474/how-to-obtain-the-first-letter-in-a-bash-variable) — kenorb, Jul 22 '18 at 23:17

fedorqui · Accepted Answer · 2016-08-18T09:24:47.593

16

To get the first character of a variable you need to say:

v="hello"
$ echo "${v:0:1}"
h

However, your code has a syntax error:

[ ! ${line:0:1} == "#"]
#                     ^-- missing space

So this can do the trick:

$ a="123456"
$ [ ! "${a:0:1}" == "#" ] && echo "doesnt start with #"
doesnt start with #
$ a="#123456"
$ [ ! "${a:0:1}" == "#" ] && echo "doesnt start with #"
$

Also it can be done like this:

$ a="#123456"
$ [ "$(expr substr $a 1 1)" != "#" ] && echo "does not start with #"
$ 
$ a="123456"
$ [ "$(expr substr $a 1 1)" != "#" ] && echo "does not start with #"
does not start with #

Update

Based on your update, this works to me:

while IFS=$'\n' read line
do
  echo $line
  if [ ! "${line:0:1}" == "#" ] # Error on this line
  then
    eval echo "$line"
    eval createSymlink $line
  fi
done < file

edited Aug 18 '16 at 09:24

answered Jul 18 '13 at 12:44

fedorqui

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Now I see doubleDown commented it. Will delete when he posts his answer! – fedorqui Jul 18 '13 at 12:51
1

I'll give you a +1 for sportsmanship too! FYI I tried this : a="123456" | [ ! "${a:0:1}" == "#" ] && echo "doesnt start with #" and got a BS error on that too. Could it be something to do with bash version? – Martyn Jul 18 '13 at 12:57
Oh I see, uhms. What about `[ "${a:0:1}" != "#" ]` ?, maybe the ` [ ! a == b ]` is not nice for your bash version. – fedorqui Jul 18 '13 at 13:00
Nice to read that, @Martyn! You can get more info about this in http://tldp.org/LDP/abs/html/refcards.html – fedorqui Jul 18 '13 at 13:04
And if @doubleDown wants to post an answer I keep my promise of deleting mine :D With +3 votes it would even make me earn a Disciplined badge! – fedorqui Jul 18 '13 at 13:04
I'm game if @doubleDown wants to play that way :) – Martyn Jul 18 '13 at 13:06
If the first character is `!` it does not work @fedorqui 'SO stop harming' – alper Apr 20 '20 at 10:57

score 4 · Answer 2 · edited May 23 '17 at 12:10

4

Adding the missing space (as suggested in fedorqui's answer ;) ) works for me.

An alternative method/syntax

Here's what I would do in Bash if I want to check the first character of a string

if [[ $line != "#"* ]]

On the right hand side of ==, the quoted part is treated literally whereas * is a wildcard for any sequence of character.

For more information, see the last part of Conditional Constructs of Bash reference manual:

When the ‘==’ and ‘!=’ operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching

Checking that you're using the right shell

If you are getting errors such as "Bad substitution error" and "[[: not found" (see comment) even though your syntax is fine (and works fine for others), it might indicate that you are using the wrong shell (i.e. not Bash).

So to make sure you are using Bash to run the script, either

make the script executable and use an appropriate shebang e.g. #!/bin/bash
or execute it via bash my_script

Also note that sh is not necessarily bash, sometimes it can be dash (e.g. in Ubuntu) or just plain ol' Bourne shell.

edited May 23 '17 at 12:10

Community

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answered Jul 18 '13 at 13:20

doubleDown

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Maybe... you're not even using Bash? – doubleDown Jul 18 '13 at 13:34
% echo $SHELL /bin/bash – Martyn Jul 18 '13 at 13:36
what do you get with `type [[`? – doubleDown Jul 18 '13 at 13:38
% type [[ [[ is a shell keyword – Martyn Jul 18 '13 at 13:39
If you're running this code in a script, maybe try adding the shebang `#!/bin/bash` to the top of it? – doubleDown Jul 18 '13 at 13:44
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/33705/discussion-between-martyn-and-doubledown) – Martyn Jul 18 '13 at 13:59
I see it was a matter of the way the script was executed, well done for finding it out! – fedorqui Jul 18 '13 at 15:34
1

@fedorqui, Yeah. I'm just glad that this didn't end up as one of those non-reproducible mystery cases. – doubleDown Jul 18 '13 at 15:44
Good! So I'm ready to delete my answer when this answer is accepted :) If @Martyn considers it necessary, of course – fedorqui Jul 18 '13 at 15:48
@fedorqui, I don't think that's necessary. Our answers are not exactly alike. – doubleDown Jul 18 '13 at 15:50

score 2 · Answer 3 · answered Jul 18 '13 at 12:57

2

Try this:

while IFS=$'\n' read line
do
  if ! [ "${line:0:1}" = "#" ]; then
    eval echo "$line"
    eval createSymlink $line
  fi
done < /some/file.txt

or you can use the following for your if syntax:

if [[ ! ${line:0:1} == "#" ]]; then

answered Jul 18 '13 at 12:57

jaypal singh

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+1 Also good point putting the negation condition outside the brakets. I am wondering what is the most POSIX way! – fedorqui Jul 18 '13 at 13:06
1

@fedorqui: '!' is a valid "not" parameter of the test ([) command, so I'd keep it inside, myself – Olivier Dulac Feb 09 '18 at 16:51

score 0 · Answer 4 · answered Feb 09 '18 at 16:58

TIMTOWTDI ^^

while IFS='' read -r line
do
  case "${line}" in
    "#"*)  echo "${line}"
    ;;
    *)     createSymlink ${line}
    ;;
  esac
done  < /some/file.txt

Note: I dropped the eval, which could be needed in some (rare!) cases (and are dangerous usually).

Note2: I added a "safer" IFS & read (-r, raw) but you can revert to your own if it is better suited. Note that it still reads line by line.

Note3: I took the habit of using always ${var} instead of $var ... works for me (easy to find out vars in complex text, and easy to see where they begin and end at all times) but not necessary here.

Note4: you can also change the test to : *"#"*) if some of the (comments?) lines can have spaces or tabs before the '#' (and none of the symlink lines does contain a '#')

How to get first character of variable

4 Answers4

Update

An alternative method/syntax

Checking that you're using the right shell