Linux Shell Programming String Compare Syntax

How to compare strings in Bash

Using variables in if statements

if [ "$x" = "valid" ]; then
  echo "x has the value 'valid'"
fi

If you want to do something when they don't match, replace = with !=. You can read more about string operations and arithmetic operations in their respective documentation.

Why do we use quotes around $x?

You want the quotes around $x, because if it is empty, your Bash script encounters a syntax error as seen below:

if [ = "valid" ]; then

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Non-standard use of == operator

Note that Bash allows == to be used for equality with [, but this is not standard.

Use either the first case wherein the quotes around $x are optional:

if [[ "$x" == "valid" ]]; then

or use the second case:

if [ "$x" = "valid" ]; then

Linux shell programming string compare syntax

These pages explain the various comparison operators in bash:

• http://www.tech-recipes.com/rx/209/bournebash-shell-scripts-string-comparison/
• http://tldp.org/LDP/abs/html/comparison-ops.html
• http://www.faqs.org/docs/Linux-HOWTO/Bash-Prog-Intro-HOWTO.html#ss11.2

On the second linked page, you will find:

==

    is equal to

    if [ "$a" == "$b" ]

    This is a synonym for =.

How do I compare two string variables in an 'if' statement in Bash?

For string equality comparison, use:

if [[ "$s1" == "$s2" ]]

For string does NOT equal comparison, use:

if [[ "$s1" != "$s2" ]]

For the a contains b, use:

if [[ $s1 == *"$s2"* ]]

(and make sure to add spaces between the symbols):

Bad:

if [["$s1" == "$s2"]]

Good:

if [[ "$s1" == "$s2" ]]

Linux Shell Script - String Comparison with wildcards

When using == or != in bash you can write:

if [[ $t1 == *"$t2"* ]]; then
    echo "$t1 and $t2 are equal"
fi

Note that the asterisks go on the outside of the quotes and that the wildcard pattern must be on the right.

For /bin/sh, the = operator is for equality only, not pattern matching. You can use case for pattern matching though:

case "$t1" in
    *"$t2"*) echo t1 contains t2 ;;
    *) echo t1 does not contain t2 ;;
esac

If you're specifically targeting Linux, I would assume the presence of /bin/bash.

bash script syntax to compare strings as integers

Put spaces around command names like [[ and [.

For example:

if [[ "$ITEMCOUNT" -eq "$ONE" ]]
then

or, if you like semicolons:

if [[ "$ITEMCOUNT" -eq "$ONE" ]]; then

And:

if [ $DIRCOUNT == "1" ]; then

or (better use of quotes):

if [ "$DIRCOUNT" == "1" ]; then  # Or just 1 (no quotes around it)

Because these are commands, you need spaces around the components of the expression too (as you have them already). Don't skimp on spaces in shell scripts (but also don't use them where they are not allowed, such as around the = in a variable assignment).

Note that both the [[ command and the == operator for [ are Bash extensions compared to the POSIX shell.

Shell equality operators (=, ==, -eq)

= and == are for string comparisons

-eq is for numeric comparisons

-eq is in the same family as -lt, -le, -gt, -ge, and -ne

== is specific to bash (not present in sh (Bourne shell), ...). Using POSIX = is preferred for compatibility. In bash the two are equivalent, and in sh = is the only one that will work.

$ a=foo
$ [ "$a" = foo ]; echo "$?"       # POSIX sh
0
$ [ "$a" == foo ]; echo "$?"      # bash-specific
0
$ [ "$a" -eq foo ]; echo "$?"     # wrong
-bash: [: foo: integer expression expected
2

(Note: make sure to quote the variable expansions. Do not leave out the double-quotes above.)

If you're writing a #!/bin/bash script then I recommend using [[ instead. The double square-brackets [[...]] form has more features, a more natural syntax, and fewer gotchas that will trip you up. For example, double quotes are no longer required around $a:

$ [[ $a == foo ]]; echo "$?"      # bash-specific
0

See also:

• What's the difference between [ and [[ in Bash?

Compare String with list of strings in bash

Assuming the az ml command returns a json array string and you want to
check if the array includes the value of variable SERVNAME, would you
please try:

SERVNAME="ner"
SERVICE='[ "ner", "aks-gpu-ner-0306210907", "aks-gpu-ner-30012231", "aks-gpu-ner-1305211336"]'

if [[ $SERVICE =~ "\"$SERVNAME\"" ]]; then
    echo "Service Found"
    # put your command here to update the service
else
    echo "Service Not Found"
    # put your command here to deploy new service
fi

The regex operator $SERVICE =~ "\"$SERVNAME\"" matches if the string $SERVICE
contains the substring $SERVNAME enclosed with double quotes.

If jq is available, you could also say:

result=$(echo "$SERVICE" | jq --arg var "$SERVNAME" '. | index($var)')
if [[ $result != "null" ]]; then
    echo "Service Found"
else
    echo "Service Not Found"
fi

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