I want to add this command grep -co '\b5\b' $INFILE in a bash script. The problem is that instead of 5 i want to use a variable in its place so i write:
V=`grep -co '\b$L\b' $INFILE`
but it does not work since the $ is used to describe internally the end of line in grep. How can i make it work? Is there an escape sequence for $ to make it use its bash meaning of the value of a variable?
Use " instead of ' to allow expansion of your $L variable.
grep "foo\$" (although that's not strictly necessary if there's nothing but a quote after the dollar sign).I regularly encounter this issue: accurately using grep or rg (ripgrep) to find $variables.
Conclusion (best practice, based on following experiments):
Use single quotes, and \-escape the $
'\$variable'
not
"\$variable' or unescaped "$variable"
## ----------------------------------------
[victoria@victoria]$ which grep
/usr/bin/grep
[victoria@victoria]$ grep --version
grep (GNU grep) 3.7
...
[victoria@victoria]$ which rg
/usr/bin/rg
[victoria@victoria]$ rg --version
ripgrep 13.0.0
...
[victoria@victoria]$
## ----------------------------------------
## RIPGREP (rg):
## -------------
## ----------------------------------------
## -i : ignore case
## -n : show matching lines, line numbers
## -l : show matching files
## -R : search recursively (grep)
## TRUE: 8 files matching '\$username':
[victoria@victoria]$ rg * --color=always -il -e '\$username' | wc -l
8
## TRUE: In those 8 files, 34 lines matching '\$username':
[victoria@victoria]$ rg * --color=always -in -e '\$username' | wc -l
34
## First 5 matching lines:
[victoria@victoria]$ rg * --color=always -in -e '\$username' | head -n5
sql/old/mysql-pg_auth-create_tables-2022.05.28.php:13:$username = "root";
sql/old/mysql-pg_auth-create_tables-2022.05.28.php:25:
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
sql/old/mysql-pg_auth-create_tables-2022.05.28.php:58:
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
src/auth.php:23:* @param string $username
src/auth.php:36:function register_user(string $email, string $username,
string $password, string $activation_code, int $expiry = 1 * 24 * 60 *
60, bool $is_admin = false): bool
## ----------------------------------------
## single quotes, unescaped $ - no matching lines
## (FALSE - there are 34 lines matching: $username):
[victoria@victoria]$ rg * --color=always -in -e '$username' | wc -l
0
## ----------------------------------------
## double quotes, \-escaped $ - no matching files
## (FALSE - there are 8 files matching: $username):
[victoria@victoria]$ rg * --color=always -il -e "\$username" | wc -l
0
## ----------------------------------------
## FALSE - as shown in following query ...
[victoria@victoria]$ rg * --color=always -il -e "$username" | wc -l
154
## ... unescaped "$username" falsely matching lines:
[victoria@victoria]$ rg * --color=always -in -e "$username" | head -n5
vendor/autoload.php:1:<?php
vendor/autoload.php:2:
vendor/autoload.php:3:// autoload.php @generated by Composer
vendor/autoload.php:4:
vendor/autoload.php:5:if (PHP_VERSION_ID < 50600) {
[victoria@victoria]$
## ----------------------------------------
## GREP
## ----
## grep is somewhat more forgiving:
## unquoted variable name (misses matches;
## should be 8 matching files):
[victoria@victoria]$ grep --color=always -ilR $username * | wc -l
3
## "$username" - FALSE (way too many files):
[victoria@victoria]$ grep --color=always -ilR "$username" * | wc -l
442
## '$username' - TRUE number of matched files:
[victoria@victoria]$ grep --color=always -ilR '$username' * | wc -l
8
[victoria@victoria]$ grep --color=always -inR '$username' * | head -n5
docs-private/_readme-victoria.txt:159: function register_user(string
$email, string $username, string $password, string $activation_code,
int $expiry = 1 * 24 * 60 * 60, bool $is_admin = false): bool
docs-private/_readme-victoria.txt:166:
$statement->bindValue(':username', $username);
sql/create_drop_mysql_tables.php:20:$username = "root";
sql/create_drop_mysql_tables.php:35: $conn = new
PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
sql/create_drop_mysql_tables.php:72: $conn = new
PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
## '\$username' - TRUE number of matched files:
[victoria@victoria]$ grep --color=always -ilR '\$username' * | wc -l
8
## "\$username" = TRUE number of matched files:
[victoria@victoria]$ grep --color=always -ilR "\$username" * | wc -l
8
