Integer expression expected error in shell script

You can use this syntax:

#!/bin/bash

echo " Write in your age: "
read age

if [[ "$age" -le 7 || "$age" -ge 65 ]] ; then
    echo " You can walk in for free "
elif [[ "$age" -gt 7 && "$age" -lt 65 ]] ; then
    echo " You have to pay for ticket "
fi

./bilet.sh: line 6: [: 7]: integer expression expected

Be careful with " "

./bilet.sh: line 9: [: missing `]'

This is because you need to have space between brackets like:

if [ "$age" -le 7 ] -o [ "$age" -ge 65 ]

look: added space, and no " "

If you are using -o (or -a), it needs to be inside the brackets of the test command:

if [ "$age" -le "7" -o "$age" -ge " 65" ]

However, their use is deprecated, and you should use separate test commands joined by || (or &&) instead:

if [ "$age" -le "7" ] || [ "$age" -ge " 65" ]

Make sure the closing brackets are preceded with whitespace, as they are technically arguments to [, not simply syntax.

In bash and some other shells, you can use the superior [[ expression as shown in kamituel's answer. The above will work in any POSIX-compliant shell.

This error can also happen if the variable you are comparing has hidden characters that are not numbers/digits.

For example, if you are retrieving an integer from a third-party script, you must ensure that the returned string does not contain hidden characters, like "\n" or "\r".

For example:

#!/bin/bash

# Simulate an invalid number string returned
# from a script, which is "1234\n"
a='1234
'

if [ "$a" -gt 1233 ] ; then
    echo "number is bigger"
else
    echo "number is smaller"
fi

This will result in a script error : integer expression expected because $a contains a non-digit newline character "\n". You have to remove this character using the instructions here: How to remove carriage return from a string in Bash

So use something like this:

#!/bin/bash

# Simulate an invalid number string returned
# from a script, which is "1234\n"
a='1234
'

# Remove all new line, carriage return, tab characters
# from the string, to allow integer comparison
a="${a//[$'\t\r\n ']}"

if [ "$a" -gt 1233 ] ; then
    echo "number is bigger"
else
    echo "number is smaller"
fi

You can also use set -xv to debug your bash script and reveal these hidden characters. See https://www.linuxquestions.org/questions/linux-newbie-8/bash-script-error-integer-expression-expected-934465/