![]() Let's say that you wanted to use the 'grep' command to find all lines in this file that contain the word 'Hello' followed by the word 'World'. ![]() Here is a file containing a few lines of text that we'll place inside the file 'hello.txt': Hello World. Therefore, if you try to do a grep search like this, you'll be waiting a long time (forever). In other words, it's waiting for you to type in some text by hand so it can search it for your term. In this case, grep will just sit there and do nothing because it's waiting for you to type some input (until CTRL+d is pressed) since nothing was fed in from stdin. Or, directly from stdin like this when no files are specified: echo " something " | grep " test "Ī common mistake is to forget to specify a file name at all when issuing a grep search when there is no input from stdin: grep " test " The grep command can accept input from two different methods: 1) From one or more files, like this: grep " test " file.txt (Added on ) By popular demand, many people suggested that I add this item as #0 to this list. The root causes of these mistakes are: Not knowing what flavour of regular expression that grep is currently using (and/or not understanding what features that flavour supports) Not considering the escaping rules of your shell Issues with character encodings. I have chosen to document these mistakes in detail because they are mistakes that beginners are very likely to make at some point, but also to not be able to debug on their own. In this article, I'll discuss 5 very confusing mistakes that have caused me to waste significant amounts of time when using the command-line tool known as 'grep' to search for things. or at least to stop relying on the shell to pass the literal value of the pattern to grep when it does not match - although bash does so by default, other shells (zsh in particular) don't.The Most Confusing Grep Mistakes I've Ever Made - By Robert Elder Introduction.Or if you want to output only matching installed packages for example dpkg -l | awk '$1 = "ii" & $2 ~ /^mos/' However I'd suggest using awk instead of grep for this task, since you can match against specific fields ex. ![]() Ii most 5.0.0a-4 amd64 Pager program similar to more and less *mos to match two arbitrary status characters anchored to start-of-line, followed by one or more spaces before mos: $ dpkg -l | grep '^. However ^ anchors a regular expression to the start of a line - and dpkg -l outputs the package's status before its name, so ^mos will never match in this context. * is superfluous - ^mos would do as well as ^mos.* However since grep by default outputs the whole line even when the match is partial, a trailing. ![]() The regex equivalent of the shell wildcard * would be. * in a regular expression is a quantifier rather than a wildcard, so mos* means m o then zero or more s characters. You should also get into the habit of quoting any pattern that you pass to grep, so that the shell does not try to expand it to a list of filenames in the current directory. Although ^mos* is a perfectly cromulent grep Basic regular Expression (BRE), it likely doesn't mean what you think it means.
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