From fe4c825cdeca60b04f0aa6ec4e754e90d22d7663 Mon Sep 17 00:00:00 2001 From: Lynne Williams Date: Sun, 17 Feb 2013 12:02:42 -0500 Subject: [PATCH] Update shell_cheatsheet.md --- shell_cheatsheet.md | 32 ++++++++++++++++---------------- 1 file changed, 16 insertions(+), 16 deletions(-) diff --git a/shell_cheatsheet.md b/shell_cheatsheet.md index 36fcc16..95b7bf3 100644 --- a/shell_cheatsheet.md +++ b/shell_cheatsheet.md @@ -92,7 +92,12 @@ changes all the instances of the word `markdown` to `software` in the first 5 `* ## 4. How to repeat operations using a loop... +Loops assign a value in a list or counter to a variable that takes on a different value each time through the loop. +There are 2 primary kinds of loops: `for` loops and `while` loops. + ### a) For loop +For loops loop through variables in a list + for varname in list do @@ -129,19 +134,21 @@ _Note:_ Bash is zero indexed, so counting always starts at `0`, not `1`. ### b) While Loop +While loops loop through the commands until a condition is met. For example COUNTER=0 - while [ ${COUNTER} -lt 10 ]; do + while [ ${COUNTER} -lt 10 ]; do command 1 command 2 COUNTER=`expr ${COUNTER} + 1` done -where +continues the loop as long as the value in the variable COUNTER is less than 10 (incremented by 1 on each iteration of the loop). +* `while`, `do`, and `done` are keywords -#### b.1) Conditional operators +#### b.1) Commonly used conditional operators | Operator | Definition | |----------|--------------------------| @@ -153,25 +160,18 @@ where | `-le` | less than or equal to | -### b) That the loop variable takes on a different value each time through the loop... -### c) The difference between a variable's name and its value... -### d) Why spaces and some punctuation characters shouldn't be used in files' names... -### e) How to display history and re-use commands... -* **`history`** --> displays your command history to the standard output (usually the screen) -## 6. Shell Scripts -### a) How to store shell commands in a file... -### b) How to run a shell script... -### c) How to pass filenames into a shell script... +## 6. Finding Things +### a) How to select lines matching patterns in text files... +| Example command | Description | +|-----------------|-------------| +| `grep [options] day haiku.txt` | finds every instance of the string `day` in the file haiku.txt and pipes it to standard output | +| * `-E` | tells grep you will be using a regular expression. Enclose the regular expression in quotes. _Note:_ the power of `grep` comes from using regular expressions. Please see the regular expressions sheet for examples | -## 7. Finding Things -### a) How to select lines matching patterns in text files... -* **`grep [options] day haiku.txt`** --> finds every instance of the string `day` in the file haiku.txt and pipes it to standard output. - * **`-E`** --> tells grep you will be using a regular expression. Enclose the regular expression in quotes. _Note:_ the power of `grep` comes from using regular expressions. Please see the regular expressions sheet for examples * **`-i`** --> makes matching case-insensitive * **`-n`** --> limits the number of lines that match to the first n matches * **`-v`** --> shows lines that do not match the pattern (inverts the match) -- 2.26.2