⌘ Web Mechanic ⌘

Bash Scripting

head

A lot of what an administrator or programmer does is hunt through text files. These may be log files from a web server, error files from an application, or simply a large ASCII document such as a script.

You may have guessed what this command does - it shows you the top 10 lines of a text file.

head sudo.txt
SUDO(8)                      System Manager's Manual                     SUDO(8)



NAME
       sudo, sudoedit - execute a command as another user

SYNOPSIS
       sudo -h | -K | -k | -V
       sudo -v [-AknS] [-g group] [-h host] [-p prompt] [-u user]

10 is the default number of lines, but you can adjust that:

head -n25 sudo.txt
SUDO(8)                      System Manager's Manual                     SUDO(8)



NAME
       sudo, sudoedit - execute a command as another user

SYNOPSIS
       sudo -h | -K | -k | -V
       sudo -v [-AknS] [-g group] [-h host] [-p prompt] [-u user]
       sudo -l [-AknS] [-g group] [-h host] [-p prompt] [-U user] [-u user]
            [command]
       sudo [-AbEHnPS] [-C num] [-g group] [-h host] [-p prompt] [-u user]
            [VAR=value] [-i | -s] [command]
       sudoedit [-AknS] [-C num] [-g group] [-h host] [-p prompt] [-u user]
                file ...

DESCRIPTION
       sudo allows a permitted user to execute a command as the superuser or
       another user, as specified by the security policy.  The invoking user's
       real (not effective) user ID is used to determine the user name with
       which to query the security policy.

       sudo supports a plugin architecture for security policies and
       input/output logging.  Third parties can develop and distribute their own

You can also specify how many characters you want to see:

head -c53 sudo.txt
SUDO(8)                      System Manager's Manual

Don't forget you can also use this as part of a pipeline.

tail

Maybe a bit more useful, at least when looking at log files.

tail sudo.txt
DISCLAIMER
       sudo is provided AS IS and any express or implied warranties, including,
       but not limited to, the implied warranties of merchantability and fitness
       for a particular purpose are disclaimed.  See the LICENSE file
       distributed with sudo or https://www.sudo.ws/license.html for complete
       details.



Sudo 1.8.17p1                   January 19, 2016                         SUDO(8)

This command has a few more options available besides the number of lines to view:

-N
print the last N lines
-n N
print the last N lines
-n +N
print all lines EXCEPT the first N
-cN
print the last N bytes
-f
this keeps the file open and prints any APPENDED lines. Very useful. Adding '--retry' will create the file if it doesn't exist yet.
-q
puts the command in 'quiet' mode when processing more than 1 file. This doesn't print the banner above each file.

tail -n +624 sudo.txt

Sudo 1.8.17p1                   January 19, 2016                         SUDO(8)

I can't stress how useful these commands are when used in a pipeline.

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