BashFAQ - Greg's Wiki

BashFAQ - Greg's Wiki

Contents

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/output of a command in a variable?
3. How can I insert a blank character after each character?
4. How can I check whether a directory is empty or not? How do I check for any *.mpg files?
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 deal with file names containing newlines, spaces or both?
21. How can I replace a string with another string in all files?
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. Why do they suddenly 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 arguments (options) to 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)?
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.
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 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 run 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) timestamps to human-readable values?
71. How do I convert an ASCII character to its decimal (or hexadecimal) value and back?
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?
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?
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 $(...) preferred over `...` (backticks)?
83. How do I determine whether a variable is already defined? Or a function?
84. How do I return a string (or large number, or negative number) from a function? "return" only lets me give a number from 0 to 255.
85. How to write several times to a fifo without having to reopen it?
86. How to ignore aliases or functions when running a command?
87. How can I get a file's permissions (or other metadata) without parsing ls -l output?
88. How can I avoid losing any history lines?
89. I'm reading a file line by line and running ssh or ffmpeg, but everything after the first line is eaten!
90. How do I prepend a text to a file (the opposite of >>)?
91. I'm trying to get the number of columns or lines of my terminal but the variables COLUMNS / LINES are always empty
92. How do I write a CGI script that accepts parameters?
93. How can I set the contents of my terminal's title bar?
94. I want to get an alert when my disk is full (parsing df output).
95. I'm getting "Argument list too long". How can I process a large list in chunks?
96. ssh eats my word boundaries! I can't do ssh remotehost make CFLAGS="-g -O"!
97. How do I determine whether a symlink is dangling (broken)?
98. How to add localization support to your bash scripts
99. How can I get the newest (or oldest) file from a directory?
100. How do I do string manipulations in bash?
101. Common utility functions (warn, die)
102. How to get the difference between two dates
103. How do I check whether my file was modified in a certain month or date range?
104. Why doesn't foo=bar echo "$foo" print bar?
105. Why doesn't set -e (set -o errexit) do what I expected?
106. I want to tee my stdout to a log file from inside the script. And maybe stderr too.