Bash Script: Read One Character At A Time

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I need to count one character at a time from input.txt. How do I read one character at a time under Linux / UNIX bash shell script?

The read builtin can read one character at a time and syntax is as follows:

read -n 1 c
echo $c

You can setup the while loop as follows:

#!/bin/bash
# data file
INPUT=/path/to/input.txt
 
# while loop
while IFS= read -r -n1 char
do
        # display one character at a time
	echo  "$char"
done < "$INPUT"

Example: Letter frequency counter shell script

#!/bin/bash
INPUT="$1"
# counter 
a=0
b=0
cc=0
 
# Make sure file name supplied
[ $# -eq 0 ] && { echo "Usage: $0 filename"; exit 1; }
 
# Make sure file exits else die 
[ ! -f $INPUT ] && { echo "$0: file $INPUT not found."; exit 2; }
 
# the while loop, read one char at a time
while IFS= read -r -n1 c
do
	# counter letter a, b, c
	[ "$c" == "a" ] && (( a++ ))
	[ "$c" == "b" ] && (( b++ ))
	[ "$c" == "c" ] && (( cc++ ))
done < "$INPUT"
 
echo "Letter counter stats:"
echo "a = $a"
echo "b = $b"
echo "c = $cc"

Run it as follows:
/tmp/readch /etc/passwd
Sample outputs:

Letter counter stats:
a = 169
b = 104
c = 39

See also:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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9 comment

  1. How about a script which runs Bash for me, but its output is slow, one-letter-at-a-time old school terminal looking? For example, when I run this script I am returned to a bash prompt; I run ”ls’ and the returning text is written as a single character every 1/2 second or so.

    Useless but I think it sounds retro and fun :)

  2. I don’t know about doing that with bash, but I think it would be cool for log files.

    #!/bin/bash
    # data file
    INPUT=/var/log/messages

    # while loop
    while IFS= read -r -n1 char
    do
    # display one character at a time
    echo -n “$char”
    sleep .05
    done < "$INPUT"

  3. Ohh! You could ALSO, include aplay atinysound.wav after the echo line, so you get that uber-cool sound when text prints.

  5. Hello Guys,
    This is great information covered here.
    When I try these examples all works perfectly except I cannot read space or tab as a character, instead I have empty $c variable.
    Do you know how to improve it to have the possiblity to read all characters including spaces and tabs?
    Cheers
    luk

  6. Yes, my mistake :)
    when the variable
    IFS=
    is set empty as above then spaces can be red succssfully.
    Cheers
    Luk

  7. The way using `read -r -n1` for reading every character is wrong, it can’t handle multi-byte characters.

    You should handle the characters in this way:

    string=”Hello world, 你好世界”
    for (( i = 0; i < ${#string}; i++ )); do
    c="${string:i:1}"
    echo $c
    # process every characters.
    done

    The downside is the performance is not pretty good if your string is very long.

  8. Would it then be slightly faster to do this ?

    until [[ "$string" == "" ]]
do
    c="${string:0:1}"; string="${string:1}";
    echo $c;
done;

  9. Problems if the input is console instead of file:
    #!/bin/bash
    INPUT=/dev/tty1
    while IFS= read -r -n1 char
    do
    echo “===$char===”
    done < "$INPUT"

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