Bash Strings

Strings in Bash

A string is a piece of text. Bash has built-in ways to change strings, using the ${...} syntax.

#!/bin/bash
first="Hello"
second="World"
greeting="$first, $second!"
echo $greeting

./join.sh
Hello, World!

String Length

Use ${#variable} to get the number of characters:

#!/bin/bash
text="Bash"
echo ${#text}

./length.sh
4

Substrings

Use ${variable:start:length} to get part of a string. The first character is at position 0:

#!/bin/bash
text="Hello World"
echo ${text:0:5}
echo ${text:6}

./sub.sh
Hello
World

If you leave out the length, you get everything from the start position to the end.

Replace Text

Use ${variable/old/new} to replace the first match, and ${variable//old/new} to replace all matches:

#!/bin/bash
text="I like cats. Cats are fun. cats!"
echo ${text/cats/dogs}
echo ${text//cats/dogs}

./replace.sh
I like dogs. Cats are fun. cats!
I like dogs. Cats are fun. dogs!

Note that "Cats" with a capital C did not change, because the match is case-sensitive.

Remove a Prefix or Suffix

Use # to remove a pattern from the start, and % to remove a pattern from the end. This is very useful for file names:

#!/bin/bash
file="report.final.txt"
echo ${file%.txt}
echo ${file%%.*}
echo ${file#*.}

./trim.sh
report.final
report
final.txt

• % removes the shortest match from the end, %% the longest
• # removes the shortest match from the start, ## the longest

Upper and Lower Case

Use ^^ to make the string upper case, and ,, to make it lower case:

#!/bin/bash
name="Bash Scripting"
echo ${name^^}
echo ${name,,}

./case.sh
BASH SCRIPTING
bash scripting

Compare Strings

Use == and != inside [[ ]] to compare strings, and -z to check if a string is empty:

#!/bin/bash
a="apple"
b="banana"
if [[ $a == $b ]]; then
  echo "Same"
else
  echo "Different"
fi
empty=""
if [[ -z $empty ]]; then
  echo "The string is empty"
fi

./compare.sh
Different
The string is empty