Programs and files 2

Repeating things and variables

Declare a variable with just its name, but refer to it with a leading $. For example:

for file in *.txt; do wc -l $file; done

The preceding declares a variable called file, which will be equal to all files in the current directory that end in .txt and then reports the number of lines in each.

In the first example below, % removes a matching string at the end of the filename (in this case, .txt), and we then append something else (in this case, .bak). This changes the extension on all .txt files to .bak; for example, it changes obdurodon.txt to obdurodon.bak. The second example keeps the .txt and appends .bak after it, changing obdurodon.txt to obdurodon.txt.bak.

Both commands as written, with cp, make copies, but if you replace cp with mv, they rename files.

for file in *.txt; do cp ${file%txt}bak; done 
for file in *.txt; do cp $file $file.bak; done

The environment

• env: show all environment variables
• $PATH: list of directories where the shell looks for executable files, starting at the beginning
• echo $PATH: show the value of the $PATH variable

More about $PATH

• Your working directory is not automatically in your path. Why might that be the case?
• To execute a file in the current directory, specify the directory as part of the command, e.g., ./command runs a command called command that’s located in the current working directory.

Finding commands and files

• help: list all shell built-ins
• which: search $PATH, one result (the one that will be executed if you type the command), not for shell built-ins or aliases
• whereis: search standard locations, multiple results, not for shell built-ins or aliases
• find / -name java -print 2> /dev/null: find all files named java starting at the file system root, not for shell built-ins or aliases
• type: includes shell built-ins and aliases!

System locale ($LOCALE and $LC_*)

Declare a variable with just it’s name, but refer to it with a leading $. Example:

 for file in *.txt; do wc -l $file; done

The preceding declares a variable called file, which will be equal to all files in the current directory that end in .txt, and then reports the number of lines in each.

• How to find your locale: the locale command
• What the different LC_* variables are for
• What happens when your locale changes: playing with date

Misc

• diff
• locale: view your locale
• diff and wdiff