pbot/applets/bashfaq.txt

1. How can I read a file (data stream, variable) line-by-line (and/or field-by-field)?
2. How can I store the return value and/or output of a command in a variable?
3. How can I sort or compare files based on some metadata attribute (newest / oldest modification time, size, etc)?
4. How can I check whether a directory is empty or not? How do I check for any *.mpg files, or count how many there are?
5. How can I use array variables?
6. How can I use variable variables (indirect variables, pointers, references) or associative arrays?
7. Is there a function to return the length of a string?
8. How can I recursively search all files for a string?
9. What is buffering? Or, why does my command line produce no output: tail -f logfile | grep 'foo bar' | awk ...
10. How can I recreate a directory hierarchy structure, without the files?
11. How can I print the n'th line of a file?
12. How do I invoke a shell command from a non-shell application?
13. How can I concatenate two variables? How do I append a string to a variable?
14. How can I redirect the output of multiple commands at once?
15. How can I run a command on all files with the extension .gz?
16. How can I use a logical AND/OR/NOT in a shell pattern (glob)?
17. How can I group expressions in an if statement, e.g. if (A AND B) OR C?
18. How can I use numbers with leading zeros in a loop, e.g. 01, 02?
19. How can I split a file into line ranges, e.g. lines 1-10, 11-20, 21-30?
20. How can I find and safely handle file names containing newlines, spaces or both?
21. How can I replace a string with another string in a variable, a stream, a file, or in all the files in a directory?
22. How can I calculate with floating point numbers instead of just integers?
23. I want to launch an interactive shell that has special aliases and functions, not the ones in the user's ~/.bashrc.
24. I set variables in a loop that's in a pipeline. Why do they disappear after the loop terminates? Or, why can't I pipe data to read?
25. How can I access positional parameters after $9?
26. How can I randomize (shuffle) the order of lines in a file? Or select a random line from a file, or select a random file from a directory?
27. How can two unrelated processes communicate?
28. How do I determine the location of my script? I want to read some config files from the same place.
29. How can I display the target of a symbolic link?
30. How can I rename all my *.foo files to *.bar, or convert spaces to underscores, or convert upper-case file names to lower case?
31. What is the difference between test, [ and [[ ?
32. How can I redirect the output of 'time' to a variable or file?
33. How can I find a process ID for a process given its name?
34. Can I do a spinner in Bash?
35. How can I handle command-line options and arguments in my script easily?
36. How can I get all lines that are: in both of two files (set intersection) or in only one of two files (set subtraction).
37. How can I print text in various colors?
38. How do Unix file permissions work?
39. What are all the dot-files that bash reads?
40. How do I use dialog to get input from the user?
41. How do I determine whether a variable contains a substring?
42. How can I find out if a process is still running?
43. Why does my crontab job fail? 0 0 * * * some command > /var/log/mylog.`date +%Y%m%d`
44. How do I create a progress bar? How do I see a progress indicator when copying/moving files?
45. How can I ensure that only one instance of a script is running at a time (mutual exclusion, locking)?
46. I want to check to see whether a word is in a list (or an element is a member of a set).
47. How can I redirect stderr to a pipe?
48. Eval command and security issues
49. How can I view periodic updates/appends to a file? (ex: growing log file)
50. I'm trying to put a command in a variable, but the complex cases always fail!
51. I want history-search just like in tcsh. How can I bind it to the up and down keys?
52. How do I convert a file from DOS format to UNIX format (remove CRs from CR-LF line terminators)?
53. I have a fancy prompt with colors, and now bash doesn't seem to know how wide my terminal is. Lines wrap around incorrectly.
53.1. Escape the colors with \[ \]
54. How can I tell whether a variable contains a valid number?
55. Tell me all about 2>&1 -- what's the difference between 2>&1 >foo and >foo 2>&1, and when do I use which?
56. How can I untar (or unzip) multiple tarballs at once?
57. How can I group entries (in a file by common prefixes)?
58. Can bash handle binary data?
59. I saw this command somewhere: :(){ :|:& } (fork bomb). How does it work?
60. I'm trying to write a script that will change directory (or set a variable), but after the script finishes, I'm back where I started (or my variable isn't set)!
61. Is there a list of which features were added to specific releases (versions) of Bash?
62. How do I create a temporary file in a secure manner?
63. My ssh client hangs when I try to logout after running a remote background job!
64. Why is it so hard to get an answer to the question that I asked in #bash?
65. Is there a "PAUSE" command in bash like there is in MSDOS batch scripts? To prompt the user to press any key to continue?
66. I want to check if [[ $var == foo || $var == bar || $var == more ]] without repeating $var n times.
67. How can I trim leading/trailing white space from one of my variables?
68. How do I run a command, and have it abort (timeout) after N seconds?
69. I want to automate an ssh (or scp, or sftp) connection, but I don't know how to send the password....
70. How do I convert Unix (epoch) times to human-readable values?
71. How do I convert an ASCII character to its decimal (or hexadecimal) value and back? How do I do URL encoding or URL decoding?
72. How can I ensure my environment is configured for cron, batch, and at jobs?
73. How can I use parameter expansion? How can I get substrings? How can I get a file without its extension, or get just a file's extension? What are some good ways to do basename and dirname?
74. How do I get the effects of those nifty Bash Parameter Expansions in older shells?
75. How do I use 'find'? I can't understand the man page at all!
76. How do I get the sum of all the numbers in a column?
77. How do I log history or "secure" bash against history removal?
78. I want to set a user's password using the Unix passwd command, but how do I script that? It doesn't read standard input!
79. How can I grep for lines containing foo AND bar, foo OR bar? Or for files containing foo AND bar, possibly on separate lines? Or files containing foo but NOT bar?
80. How can I make an alias that takes an argument?
81. How can I determine whether a command exists anywhere in my PATH?
82. Why is $(...) preferred over `...` (backticks)?
83. How do I determine whether a variable is already defined? Or a function?
84. How do I return a string (or large number, or negative number) from a function? "return" only lets me give a number from 0 to 255.
85. How to write several times to a fifo without having to reopen it?
86. How to ignore aliases or functions when running a command?
87. How can I get a file's permissions (or other metadata) without parsing ls -l output?
88. How can I avoid losing any history lines?
89. I'm reading a file line by line and running ssh or ffmpeg, only the first line gets processed!
90. How do I prepend a text to a file (the opposite of >>)?
91. I'm trying to get the number of columns or lines of my terminal but the variables COLUMNS / LINES are always empty.
92. How do I write a CGI script that accepts parameters?
93. How can I set the contents of my terminal's title bar?
94. I want to get an alert when my disk is full (parsing df output).
95. I'm getting "Argument list too long". How can I process a large list in chunks?
96. ssh eats my word boundaries! I can't do ssh remotehost make CFLAGS="-g -O"!
97. How do I determine whether a symlink is dangling (broken)?
98. How to add localization support to your bash scripts
99. How can I get the newest (or oldest) file from a directory?
100. How do I do string manipulations in bash?
101. Common utility functions (warn, die)
102. How to get the difference between two dates
103. How do I check whether my file was modified in a certain month or date range?
104. Why doesn't foo=bar echo "$foo" print bar?
105. Why doesn't set -e (or set -o errexit, or trap ERR) do what I expected?
106. Logging! I want to send all of my script's output to a log file. But I want to do it from inside the script. And I want to see it on the terminal too!
107. How do I add a timestamp to every line of a stream?
108. How do I wait for several spawned processes?
109. How can I tell whether my script was sourced (dotted in) or executed?
110. How do I copy a file to a remote system, and specify a remote name which may contain spaces?
111. What is the Shellshock vulnerability in Bash?
112. What are the advantages and disadvantages of using set -u (or set -o nounset)?
113. How do I extract data from an HTML or XML file?
114. How do I operate on IP addresses and netmasks?
115. How do I make a menu?
116. I have two files. The first one contains bad IP addresses (plus other fields). I want to remove all of these bad addresses from a second file.
117. I have a pipeline where a long-running command feeds into a filter. If the filter finds "foo", I want the long-running command to die.
118. How do I print the contents of an array in reverse order, or reverse an array?
119. What's the difference between "cmd < file" and "cat file | cmd"? What is a UUOC?
120. How can I find out where this strange variable in my interactive shell came from?
121. What does value too great for base mean? (Octal values in arithmetic.)