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Linux Filesystem Some tricks that make using the Linux command line a really useful tool. Commands for the Linux shell How to find and remove empty folders in a directory tree How to change only one specific file permission How to change ownership of file and group in one step What is the longest path on your system How to redirect outputs from command line tools Redirect only stderr to a pipe Closing File Descriptors Another cool command Command Substitution Basic arithmetic evaluation Feed continuously strings to stdout Troubleshooting Some shared objects can't be found Interacting with Linux systems How to read a ext3fs linux partition from Windows Commands for the Linux shell The following commands have been tested only with the bash shell. How to find and remove empty folders in a directory tree find-type d -empty -delete If we also want to remove empty files we just don't use the -type d option. How to change only one specific file permission chmod g+w This would only set the groups write permission. How to change ownership of file and group in one step chown username.groupname What is the longest path on your system find /| awk '{print length($0) " " $0}' | sort -n | tail -n 1 On my system this yields in: 240 /cygdrive/c/ProgramFiles/Development/Framework/eclipse/plugins/org.eclipse.platform.source_3.2.1.r321_v20060921-b_XVA-INSQSyMtx /src/org.eclipse.core.runtime.compatibility.registry_3.2.1.R32x_v20060907/ runtime_registry_compatibilitysrc.zip How to redirect outputs from command line tools These redirections are tested for the bash shell, but maybe work also with other shells. Symbol Explanation Example > stdout to file ls -l > list.txt 2> stderr to file grep da * 2> grep-errors.txt 1>&2 stdout to stderr grep da * 1>&2 2>&1 stderr to stdout grep * 2>&1 &> stdout and stderr to file rm -f $(find / -name core) &> /dev/null Redirect only stderr to a pipe Pipes normally only work with stdout, so here we show how to redirect the stderr to a pipe: exec 3>&1 # Save current "value" of stdout. ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls'). # ^^^^ ^^^^ exec 3>&- # Now close it for the remainder of the script. Closing File Descriptors n<&- Close input file descriptor n. 0<&, <& Close stdin. n>&- Close output file descriptor n. 1>&, >& Close stdout. Another cool command The following command copies stdout and stderr to the files "stdout.txt" and "stderr.txt", respectively. (((./cmd | tee stdout.txt) 3>&1 1>&2 2>&3 | tee stderr.txt) 3>&1 1>&2 2>&3) 1>out.txt 2>err.txt Command Substitution Bash uses named pipes in a really neat way. Recall that when you enclose a command in parenthesis, the command is actually run in a ``subshell''; that is, the shell clones itself and the clone interprets the command(s) within the parenthesis. Since the outer shell is running only a single ``command'', the output of a complete set of commands can be redirected as a unit. For example, the command: (ls -l; ls -l) >ls.out writes two copies of the current directory listing to the file ls.out. Command substitution occurs when you put a < or > in front of the left parenthesis. For instance, typing the command: cat <(ls -l) The command: ls | tee >(grep foo | wc >foo.count) \ >(grep bar | wc >bar.count) \ | grep baz | wc >baz.count counts the number of occurrences of foo, bar and baz in the output of ls and writes this information to three separate files. Command substitutions can even be nested: cat <(cat <(cat <(ls -l)))) works as a very roundabout way to list the current directory. Another form of command substitution is using "`": echo "`date`" If the output of the substituted command contains 0x00-bytes these will be silently purged by the shell and will never reach the outermost command! Basic arithmetic evaluation Bash even provides us with very basic arithmetic support: echo "$((155 * 345))" Feed continuously strings to stdout In its default behavior the yes command feeds a continuous string of the character y followed by a line feed to stdout. A control-c terminates the run. A different output string may be specified: yes "sample" yes $BASH_VERSION One might well ask the purpose of this. From the command line or in a script, the output of yes can be redirected or piped into a program expecting user input. Troubleshooting Some shared objects can't be found For example, you want to test a program you copied from another system with its shared object libraries, but you don't want to install these in to your paths, you can change the LD_LIBRARY_PATH variable point to these: export LD_LIBRARY_PATH=/path_where_the_so_are:$LD_LIBRARY_PATH Interacting with Linux systems How to read a ext3fs linux partition from Windows Have a look at this filesystem driver. Commands for the Linux shell How to find and remove empty folders in a directory tree How to change only one specific file permission How to change ownership of file and group in one step What is the longest path on your system How to redirect outputs from command line tools Redirect only stderr to a pipe Closing File Descriptors Another cool command Command Substitution Basic arithmetic evaluation Feed continuously strings to stdout Troubleshooting Some shared objects can't be found Interacting with Linux systems How to read a ext3fs linux partition from Windows

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