My Favorite bash Tips and Tricks
The inner command, rpm -qa | grep httpd, lists all the packages that have httpd in the name. The outer command, rpm -ql, lists all the files in each package.
Now, those of you who have experience with the Bourne shell might point out that you could perform command substitution by surrounding a command with back quotes, also called back-ticks. Using Bourne-style command substitution, the date assignment from above becomes:
today2=`date +%d-%b-%Y` $ echo $today2 12-Mar-2004
There are two important advantages to using the newer bash-style syntax for command substitution. First, it can be nested more easily. Because the opening and closing symbols are different, the inner symbols don't need to be escaped with back slashes. Second, it is easier to read, especially when nested.
Even on Linux, where bash is standard, you still encounter shell scripts that use the older, Bourne-style syntax. This is done to provide portability to various flavors of UNIX that do not always have bash available but do have the Bourne shell. bash is backward-compatible with the Bourne shell, so it can understand the older syntax.
Have you ever looked for a file using the find command, only to learn the file you were looking for is lost in a sea of permission denied error messages that quickly fill your terminal window?
If you are the administrator of the system, you can become root and execute find again as root. Because root can read any file, you don't get that error anymore. Unfortunately, not everyone has root access on the system being used. Besides, it's bad practice to be root unless it's absolutely necessary. So what can you do?
One thing you can do is redirect your output to a file. Basic output redirection should be nothing new to anyone who has spent a reasonable amount of time using any UNIX or Linux shell, so I won't go into detail regarding the basics of output redirection. To save the useful output from the find command, you can redirect the output to a file:
$ find / -name foo > output.txt
You still see the error messages on the screen but not the path of the file you're looking for. Instead, that is placed in the file output.txt. When the find command completes, you can cat the file output.txt to get the location(s) of the file(s) you want.
That's an acceptable solution, but there's a better way. Instead of redirecting the standard output to a file, you can redirect the error messages to a file. This can be done by placing a 2 directly in front of the redirection angle bracket. If you are not interested in the error messages, you simply can send them to /dev/null:
$ find / -name foo 2> /dev/null
This shows you the location of file foo, if it exists, without those pesky permission denied error messages. I almost always invoke the find command in this way.
The number 2 represents the standard error output stream. Standard error is where most commands send their error messages. Normal (non-error) output is sent to standard output, which can be represented by the number 1. Because most redirected output is the standard output, output redirection works only on the standard output stream by default. This makes the following two commands equivalent:
$ find / -name foo > output.txt $ find / -name foo 1> output.txt
Sometimes you might want to save both the error messages and the standard output to file. This often is done with cron jobs, when you want to save all the output to a log file. This also can be done by directing both output streams to the same file:
$ find / -name foo > output.txt 2> output.txt
This works, but again, there's a better way to do it. You can tie the standard error stream to the standard output stream using an ampersand. Once you do this, the error messages goes to wherever you redirect the standard output:
$ find / -name foo > output.txt 2>&1
One caveat about doing this is that the tying operation goes at the end of the command generating the output. This is important if piping the output to another command. This line works as expected:
find -name test.sh 2>&1 | tee /tmp/output2.txt