Linux: How to view log files on the shell?

Saturday, August 8, 2009 posted by Till

Many linux servers are administered on the commandline e.g. with a SSH connection. In the following article, I will explain several shell commands that make it easy to view logfiles.

The most important command is “tail”. Tail can be used to read the last lines from a file. Examples:

Get the last 100 lines from the Debian mail log file:

tail -n 100 /var/log/mail.log

To get all newly added lines from a log file in realtime on the shell, use the command:

tail -f /var/log/mail.log

to quit tail and go back to the commanline press the keys [ctrl] + [c]

If you want to get the last 1000 lines from a log file and they do not fit into your shell window, you can use the command “more” to be able to view them line by line.

tail -n 1000 /var/log/mail.log | more

press [space] to go to the next line or [ctrl] + [c] to quit.

If you want to search for a specific term in a large file, the command “grep” comes in handy. Example: We search for the email address “tom@anydomain.tld” in the mail log file:

grep “tom@anydomain.tld” /var/log/mail.log

If you want to view the whole content of file on the shell, use the command “cat”. Example:

cat /proc/cpuinfo

will show you detailed info about the CPU of your computer.

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