Grep for a string in file without using pipe

command linegreppipesearch

I want to grep for a word in a file in the last n lines without using the pipe.

grep <string> filename

enables to search the filename for a string. But, I want to search for a string in the last N lines of the file. Any command to search for that without using the pipe?

Best Answer

If your shell supports it (zsh, bash, some implementations of ksh), you could utilise process substitution

grep <pattern> <(tail -n5 yourfile.txt)

Where -n5 means get the five last lines.

Similarly,

grep <pattern> <(head -n5 yourfile.txt)

would search through the 5 first lines of yourfile.txt.

Explanation

Simply speaking, the substituted process pretends to be a file, which is what grep is expecting. One advantage with process substitution is that you can feed output from multiple commands as input for other commands, like diff in this example.

diff -y <(brew leaves) <(brew list)

This gets rid of the pipe (|) character, but each substitution is in fact creating a pipe¹.

_{¹Note that with ksh93 on Linux at least, | does not use a pipe but a socket pair while process substitution does use a pipe (as it's not possible to open a socket):}

$ ksh93 -c 'readlink <(:)'
pipe:[620224]
$ ksh93 -c ': | readlink /proc/self/fd/0'
socket:[621301]

Related Solutions

Why Doesn’t Grep Using Pipe Work?

That's nothing to do with grep - it's because the pipe | redirects the standard output stream stdout whereas the Permission denied messages are in the standard error stream stderr. You could achieve the result you want by combining the streams using 2>&1 (redirect the stream whose file descriptor is 2 to the stream whose file descriptor is 1) so that stderr as well as stdout gets piped to the input of the grep command

find / -name libGL.so.1 2>&1 | grep -v 'denied'

but it would be more usual to simply discard stderr altogether by redirecting it to /dev/null

find / -name libGL.so.1 2>/dev/null

Using |& instead of 2>&1 |

If you take a look at the Bash man page you'll likely notice this blurb:

If |& is used, the standard error of command is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |.

So you can also use this construct as well if you want to join STDERR and STDOUT:

find / -name libGL.so.1 |& grep -v 'denied'

Grep string where next line does not contain string

You can use perl compatible regular expressions grep:

$ pcregrep -M '(searchString.*\n)(?!.*excludeString)' file
foo2 searchString bar
foo3 searchString bar
foo4 searchString bar

It searches searchString followed by any char ., repeated zero or more times *, followed by new line \n only if there is not (?!) pattern .*excludeString next to it. Option -M is present in order to match multi lines.

Best Answer

Related Solutions

Why Doesn’t Grep Using Pipe Work?

Using |& instead of 2>&1 |

Grep string where next line does not contain string

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