Through the command shell we can redirect the three standard filestreams so that we can get input from either a file or another command instead of from our keyboard, and we can write output and errors to files or send them as input for subsequent commands.For example, if we have a program called do_something that reads from stdin and writes to stdout and stderr, we can change its input source by using the less-than sign ( < ) followed by the name of the file to be consumed for input data:$ do_something < input-fileIf you want to send the output to a file, use the greater-than sign (>) as in:$ do_something > output-fileBecause stderr is not the same as stdout, error messages will still be seen on the terminal windows in the above example.If you want to redirect stderr to a separate file, you use stderr’s file descriptor number (2), the greater-than sign (>), followed by the name of the file you want to hold everything the running command writes to stderr:$ do_something 2> error-fileA special shorthand notation can be used to put anything written to file descriptor 2 (stderr) in the same place as file descriptor 1 (stdout): 2>&1$ do_something > all-output-file 2>&1bash permits an easier syntax for the above:$ do_something >& all-output-fileEND
PipesThe UNIX/Linux philosophy is to have many simple and short programs (or commands) cooperate together to produce quite complex results, rather than have one complex program with many possible options and modes of operation. In order to accomplish this, extensive use of pipes is made; you can pipe the output of one command or program into another as its input.In order to do this we use the vertical-bar, |, (pipe symbol) between commands as in: $ command1 | command2 | command3The above represents what we often call a pipeline and allows Linux to combine the actions of several commands into one. This is extraordinarily efficient because command2 and command3 do not have to wait for the previous pipeline commands to complete before they can begin hacking at the data in their input streams; on multiple CPU or core systems the available computing power is much better utilized and things get done quicker. In addition there is no need to save output in (temporary) files between the stages in the pipeline, which saves disk space and reduces reading and writing from disk, which is often the slowest bottleneck in getting something done.END
Being able to quickly find the files you are looking for will make you a much happier Linux user! You can search for files in your parent directory or any other directory on the system as needed.In this section, you will learn how to use the locate and find utilities, and how to use wildcards in bash.END
The locate utility program performs a search through a previously constructed database of files and directories on your system, matching all entries that contain a specified character string. This can sometimes result in a very long list.To get a shorter more relevant list we can use the grep program as a filter; grep will print only the lines that contain one or more specified strings as in: $ locate zip | grep binwhich will list all files and directories with both "zip" and "bin" in their name . (We will cover grep in much more detail later.) Notice the use of | to pipe the two commands together.Locate utilizes the database created by another program, updatedb. Most Linux systems run this automatically once a day. However, you can update it at any time by just running updatedb from the command line as the root user.END
You can search for a filename containing specific characters using wildcards.END
find is extremely useful and often-used utility program in the daily life of a Linux system administrator. It recurses down the filesystem tree from any particular directory (or set of directories) and locates files that match specified conditions. The default pathname is always the present working directory.For example, administrators sometimes scan for large core files (which contain diagnostic information after a program fails) that are more than several weeks old in order to remove them. It is also common to remove files in /tmp (and other temporary directories, such as those containing cached files) that have not been accessed recently. Many distros use automated scripts that run periodically to accomplish such house cleaning.END
When no arguments are given, find lists all files in the current directory and all of its subdirectories. Commonly used options to shorten the list include -name (only list files with a certain pattern in their name), -iname (also ignore the case of file names), and -type (which will restrict the results to files of a certain specified type, such as d for directory, l for symbolic link or f for a regular file, etc). Searching for files and directories named "gcc":$ find /usr -name gccSearching only for directories named "gcc":$ find /usr -type d -name gccSearching only for regular files named "test1":$ find /usr -type f -name test1Click the image to view an enlarged version.END
Another good use of find is being able to run commands on the files that match your search criteria. The -exec option is used for this purpose.To find and remove all files that end with .swp:$ find -name "*.swp" -exec rm {} ’;’The {} (squiggly brackets) is a place holder that will be filled with all the file names that result from the find expression, and the preceding command will be run on each one individually.Note that you have to end the command with either ‘;’ (including the single-quotes) or \; Both forms are fine.One can also use the -ok option which behaves the same as -exec except that find will prompt you for permission before executing the command. This makes it a good way to test your results before blindly executing any potentially dangerous commands.
It is sometimes the case that you wish to find files according to attributes such as when they were created, last used, etc, or based on their size. Both are easy to accomplish.Finding based on time:$ find / -ctime 3Here, -ctime is when the inode meta-data (i.e., file ownership, permissions, etc) last changed; it is often, but not necessarily when the file was first created. You can also search for accessed/last read (-atime) or modified/last written (-mtime) times. The number is the number of days and can be expressed as either a number (n) that means exactly that value, +n which means greater than that number, or -n which means less than that number. There are similar options for times in minutes (as in -cmin, -amin, and -mmin).Finding based on sizes:$ find / -size 0Note the size here is in 512-byte blocks, by default; you can also specify bytes (c), kilobytes (k), megabytes (M), gigabytes (G), etc. As with the time numbers above, file sizes can also be exact numbers (n), +n or -n. For details consult the man page for find.For example, to find files greater than 10 MB in size and running a command on those files:$ find / -size +10M -exec command {} ’;’END